Please double check the speed/prop calculator info...does it seem right to you?

[kentuckydiesel]

I had been having some ventilation issues but after some adjusting the steering link between my two outboards and adjusting the tilt, I have the ventilation under control.

Still having issues with the engines overpowering the props. Trouble is, with four people in the boat, they are still overpowering the props so much that it's hard to pin down how much prop I actually need.

Engines are 3cyl 70hp Johnsons, one a 1974, the other a 1981...Props are 13.25"x19". Boat is a 1961 lone star 23' with a warped planing hull (sharp deadrise at the bow, "nearly" flat in the stern). Weight of the boat/outboards should be around 1500lbs. I went ahead and figured on adding 1000lbs of people and gear just to be conservative...though the boat would likely be running around 2000lbs most of the time.

Look at these numbers:


According to this calculator, with the boat weighed down with 1000lbs of people and gear, I should be able to hit 59mph with the 140hp generated by the two 70hp outboards.

I am currently having to be very careful not to over rev the engines because the props aren't loading them enough, so this seems correct.



Now the prop calculation. BTW, I entered the 59mph into both speed and theoretical speed...though I am almost positive that I need to use actual speed.



This is basically telling me that I need a prop with a 27"-30" pitch.

While that would be fine with me...I can't seem to find any props available for the johnson 70s over 23" pitch. Anyone know if that might be because the little trim tabs are in the way?


What do you guys think? Does this all seem logical? Remember, even though there are alot of 23' boats out there with more hp running less max speed, this is a very light dry hull.

Thanks,
Phillip

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I compared this to 2 130 hp singles with 2.25 gear ratio one Glass at 3500 lbs all up 17" prop at 39 mph and 5700 rpm.
the other at 2640 Al. with 17" prop at 5625 rpm and 38 mph. The lighter aluminum boat slower than the smoother glass boat.
If your motors Are healthy they are probably 130 hp at the prop like the Evinrude I checked against the weights I figured.
Your motors weigh about 440 Your boat when built was 1200 lbs.The single was 405. It appears to me that your rig would produce similar numbers.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I compared this to 2 130 hp singles with 2.25 gear ratio one Glass at 3500 lbs all up 17" prop at 39 mph and 5700 rpm.
the other at 2640 Al. with 17" prop at 5625 rpm and 38 mph. The lighter aluminum boat slower than the smoother glass boat.
If your motors Are healthy they are probably 130 hp at the prop like the Evinrude I checked against the weights I figured.
Your motors weigh about 440 Your boat when built was 1200 lbs.The single was 405. It appears to me that your rig would produce similar numbers.

My boat was 1200 when built, but since then, the cabin and cabin interior have been removed to make the boat a open bow/walk-through design. Should weigh around 1000lbs now...or 1500lbs with the engines and fuel. With just my wife and I on the boat with a little bit of gear, I'm sure it would weigh in below 2000lbs.

At first glance, it seemed odd to me that you were getting 39mph on the glass boat with a 17" prop. I input the numbers into a slip calculator and it said that would be 3% slip. Is 3% slip even possible?
The Aluminum at 38mph says 6% slip which is also very low.

Have you tried more pitch on this motor with either hull? Given that the 17" prop is completely maxed out on speed, I would bet that the same engine with a higher pitch prop would propel the lighter aluminum hull a good bit faster.

Another guy with this same hull (but still set up as a cabin cruiser) had this to say:

Had the boat on the water several times now, somehow a 90 hp motor makes a 23 foot boat go 35 mph, haven't figured that one out yet. I think i can get a bit more with the right prop and a tune up though. It handles really well though, turns and manuevers better than a modern boat. Its interesting because it doesn't really get on plane like a normal boat, the whole boat just hops out of the water and skims along on top of it on its really flat bottom. When you turn it just digs in like turning a surf board which makes it able to do some really odd manuevers.

I wish I could talk to this guy and see if he ever got more speed with the 90hp Johnson V4 he was running...but from what I can tell, that info was posted here ( 1958 23' Lone Star Cruise Liner - Top speed 35 mph. - Slideshow View ) in 2005, and I doubt he could be found today.

Thanks,
Phillip

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

So you stripped out the cabin and interior and didn't add a floor, fishing platform seats or devices like a live well
trolling motor ,batteries etc. The tests were on the Evinrude etec site And I believe props and yes those slip numbers are low.
A 18 ft alumacraft with an al. prop I got to 6% low but not way out. The other was a 21 ft.? Carolina skiff with a Viper prop.
It is said these cupped props test an inch higher, that nets about 7%
From my limited research many times an aluminum boat is slower than a similar but usually heavier glass boat.
While light I don't think it is a particularly fast bottom with three keels and all those rivets and seams.
Point is get the 17" so your sure they are not venting. before adding props.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

So you stripped out the cabin and interior and didn't add a floor, fishing platform seats or devices like a live well
trolling motor ,batteries etc. The tests were on the Evinrude etec site And I believe props and yes those slip numbers are low.
A 18 ft alumacraft with an al. prop I got to 6% low but not way out. The other was a 21 ft.? Carolina skiff with a Viper prop.
It is said these cupped props test an inch higher, that nets about 7%
From my limited research many times an aluminum boat is slower than a similar but usually heavier glass boat.
While light I don't think it is a particularly fast bottom with three keels and all those rivets and seams.
Point is get the 17" so your sure they are not venting. before adding props.

I didn't strip it...it came to me that way. Basically, the roof, back cabin wall, sink, heavy porcelain toilet, etc, etc, etc, were removed. Now it's just a couple back to back seats and a couple pedestal seats, walkthrough windshield, and dash areas on either side. It does have floors though. No live wells or anything of that nature.

Those boats you mentioned are good examples. I appreciate the info.

That said, I think I'm in a bit different range than the example 130hp engine since mine have a 2.42 gear ratio rather than the 2.25 gears. Not a huge difference, but a difference nonetheless.
I'll have to check and see what the difference in prop pitch should be (if much of anything at all), given the difference in gear ratio.



-Phillip

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Hmmm...here's another test on a mercury 115 with a 13x19 prop...aluminum hull, weight about the same...45.2mph.

...and their gear ratio is 2.07:1

Engine Test | Mercury Marine

Now I'm just not sure. Wish these companies would give data on a few different props.


-Phillip

On edit:

Also, a Lund with a 13.25x20 prop that did 47.5mph.

http://www.mercurymarine.com/engines/engine-tests/boat-house-bulletin/?ID=489&

 

[emilsr]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I can't quite wrap my head around a 23' aluminum boat that only weighs 1200 lbs. How certain are you of this weight? Have you put it on the scales?

The bigger issue though is the constant you're using is way too high. That particular type of hull is going to be in the 160-170 range. Run the numbers again with the correct constant and you'll get a more accurate speed estimate.

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

The 1200 is from the brochure of that year. My guess is just the boat no bunks,head,water storage etc.
Probably available as options.

 

[jestor68]

Re: Please double check the speed/prop calculator info...does it seem right to you?

That speed calculator is more for high performance off shore type boats than your aluminum boat.

Can you get a WOT rpm reading now that you have the ventilation issue under control?

It should be noted that twin engines typically run up to 4 inches more pitch than a single engine of like power.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I can't quite wrap my head around a 23' aluminum boat that only weighs 1200 lbs. How certain are you of this weight? Have you put it on the scales?

The 1200 is from the brochure of that year. My guess is just the boat no bunks,head,water storage etc.
Probably available as options.

Just checked again, they said 1200lbs with with all the interior/bunks, sliding windows, windshield, galley assembly with sink and pump, toilet compartment (but no toilet), steering, padded transom seat, Helmsman's platform with 11gal gas storage, lighting, etc.

I should run my boat across the scales, then take the empty trailer through so I can find out for sure.

The bigger issue though is the constant you're using is way too high. That particular type of hull is going to be in the 160-170 range. Run the numbers again with the correct constant and you'll get a more accurate speed estimate.

Thanks...I will try running that again with the lower constant.

Can you get a WOT rpm reading now that you have the ventilation issue under control?

Not really. The motor is supposed to have a max operating RPM of about 5500. I have pushed it a little farther to see if I had a top rpm number, but I pulled it back after it hit around 6500rpm. There was definitely more rpm to be had...and that was with 4 people in the boat.

That said, I'm still getting something like 30% slip at 5000-5500rpm. May have something to do with prop wear?

My whole reason for trying to use one of these boat speed calculators is that my props aren't doing everything that 19" props should, so I can't really go off of the speed/rpm I'm seeing with them.
Assuming 10% slip, I should be hitting 37mph at 5500rpm...but I'm down around 25-30mph most of the time.


Thanks,
Phillip

 

[steelespike]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I just noticed you are running 13.25 X 19 props.I think either these props are wrong or you have the oversized gear case
which I think is a different ratio. A very brief check for the 70 to 1983 I don't think any were over 12" and
Usually as pitch goes up diameter goes down. the only 19 listed was 11/3/4" All the 17" props were 11 3/4 or 12".
The commercial version (larger gear case) the only 19" props under 13" were the 4 blades and that is normal.
The rest were all 13-13.25. Oversize props on a standard gear case may have a problem with exhaust gasses fouling the prop.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

There were two gear cases. From what I have seen on websites selling props, they all seem to list:
13 spline, through hub exhaust, large gearcase (1974-2001)
and
13 spline, through hub exhaust, small gearcase (1974-1983)

From what I understand, the 13"+ props physically will not fit on the small gear case outboards.

In addition to the props that are on it, I was given a set of 13 3/4"x15" props. They also fit with a good 1/2" of clearance.

Thanks,
Phillip

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Also, mine have the 3 hole water intake, where the small gear case has 4 water intake holes.

The info I can find says that both the small gear case and large gear case have 2.42:1 ratio.

http://www.sterndrive.info/outboardmotor/id1.html

Thanks,
Phillip

 

[emilsr]

Re: Please double check the speed/prop calculator info...does it seem right to you?

That speed calculator is more for high performance off shore type boats than your aluminum boat.

Respectfully, this is absolutely not true as it applies to any planing hull.

The formula was developed by a naval architect (George Crouch if you want to Google) in the 20's...way before there was any such thing as a high performance offshore boat. The constant accounts for the performance of the boat; higher performing boats having a higher number. Back in those days, a constant of 150-160 was pretty good with race boats approaching 200. Today a good monohull race boat is 250+, and the better racing cats approaching 300. Technology marches on, but the basic math remains the same.

Phillip; You're probably going to wind up with a slip number in the 20% range when all is working as it should. Don't get TOO wrapped up in theories and formulas though as the only accurate way to prop a boat is with testing. Weight is what it is, it matters when you're doing the math but not so much in real world testing. Data accuracy is critical; 'garbage in, garbage out'. I'd worry more about the accuracy of the tachs. If you haven't already, check them to make sure what you think you're seeing is actually what you're seeing.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Okay,
After using the lower constant, I should still be able to hit the high 40s or low 50s

I did fire the engines up on the muffs yesterday evening and checked the tachs using a handheld mechanical tach. They seem to be right where they should be. Unfortunately my mechanical tach only goes up to 3500rpm so I couldn't check the upper end of the RPM range, but I didn't really want to do that with no load on the engines anyway.

Thanks,
Phillip

 

[emilsr]

Re: Please double check the speed/prop calculator info...does it seem right to you?

Okay,
After using the lower constant, I should still be able to hit the high 40s or low 50s

I doubt you'll see anywhere near those speeds.

I came up with a theoretical speed of 40mph (2500lbs, 140 hp, 170 constant). 32mph with 20% prop slip.

With a 2.42 drive ratio, you're looking at 20-21" of pitch.

Are your current props as old as the motors? If so, and they're made of aluminum, it's very likely they've flattened out a bit and the actual pitch isn't what is stated on the prop. (just throwing that out there)

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I can't be sure about the age of these props, but as I was saying in a previous post, I can't really base anything off of these props because they have flattened out some, and are slipping far too much.

That said, last time I tried to see if I could get a reading on WOT RPM, The engines quickly ran up to 6500 rpm...so I just backed out on the throttles. They would have passed 6500 and kept on going if I hadn't pulled it back.

In that little bit of time before I pulled the throttles back, the boat had shot from about 25mph to around 35mph at 6500...and that's still 28% slip on a 19" pitch prop. Given that the engines are still overpowering the props like crazy at 35mph, wouldn't you think that they would take the boat well beyond 35 when "hooked up".
I imagine it to be like a car which can't hold traction at 55mph because it's overpowering the tires/suspension. Now, if you set that car up as it should be and get it to keep traction, it will obviously go well beyond 55mph.

I have to say, this boat rides a much differently than I'm used to. I'm mostly accustomed to larger boats such as the 28' Carver Express Cruiser that my parents used to own, the 38' Cruisers that my cousin used to own, and assorted big center consoles belonging to friends. All those boats always felt very much "IN" the water when on plane. This boat on the other hand, almost feels like riding on a skipping stone when you cross about 25mph. Not that it's pounding, but it almost feels like you're just above the water, touching down just every so often. It's strange. The first time I ever made turn at 25+mph, I expected it to "slide" through the turn, but somehow, it carves right through, even when turning hard. It's just a very different thing than I'm used to...but I mean that in a good way.

-Phillip

 

[emilsr]

Re: Please double check the speed/prop calculator info...does it seem right to you?

In that little bit of time before I pulled the throttles back, the boat had shot from about 25mph to around 35mph at 6500...and that's still 28% slip on a 19" pitch prop. Given that the engines are still overpowering the props like crazy at 35mph, wouldn't you think that they would take the boat well beyond 35 when "hooked up".

The problem with that theory is we're assuming the 19" props still have 19" of pitch. Do the math for 16 or 17 inches of pitch and the results will be different.

Where are you at in Kentucky? There's a few prop shops around that will let you test and/or swap props until you get the right ones. The boat dealer in Elizabethtown had a ton of props in the shop when we lived there. Honestly, I think some good testing is going to be your best bet. All these formulas and calculators (and internet experts) are okay, but the best they (we) can do is get you in the neighborhood. To be WHERE you need to be requires testing....on any boat. You will need accurate data measurement though, so you might want to beg/borrow/steal a shop tach that you can run on the water at WOT as well as a handheld GPS. Don't assume anything/verify everything.

 

[kentuckydiesel]

Re: Please double check the speed/prop calculator info...does it seem right to you?

I'm about 45min North East of Louisville. E-town is a pretty good drive from here, and I sure don't know of any nearby places that will let me try props...otherwise I wouldn't even be bothering you fine people with this thread. It's one of those *** backward situations at this point, because I don't really know of anyone running this hull, my props are worn out, and I hate to buy two new ones just to find out they aren't what I need. (actually, I wouldn't care in the slightest, but my wife is a redhead, with all the temper they're known for, which means possible death if I screw anything up.)

Thanks,
Phillip

 

[emilsr]

Re: Please double check the speed/prop calculator info...does it seem right to you?

You might try calling some shops in Louisville/Jeffersonville. I bet you'll find someone that will let you buy and try. Its an interesting project; someone's curiosity will get the best of them on this deal.

Best of luck! Please post up your results.