prop + more hp = more speed @ same rpm?

[MikDee]

Re: prop + more hp = more speed @ same rpm?

Oriole, You are trimming the drive up slowly at WOT, Right? Till almost the point of cavitation, then down a touch till it grabs fully! That's how it's done to attain highest speed.

 

[hwsiii]

Re: prop + more hp = more speed @ same rpm?

Oriole, you are asking a lot of questions and can't figure out what is going on. What we all need is all of the information on the boat and motor, everybody is trying to figure out what your problem is but yet they don't have the correct information to help you with.
The following form has all of the information that is needed to help you with your problems finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planning hull boat that needs this information, and anyone else who is trying to help you will need the same information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the exact prop you need for your particular uses, and this information is required to do that.
It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor
8. Manufacturer?s recommended WOT range
9. Antiventilation Plate height above keel of boat if it is an outboard in inches
10. Make, model, diameter, pitch and whether SS or aluminum prop
11. WOT RPM and speed from each prop

H

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

1 1988 Starline Aggressor (not many have heard of these)
2 16'3" long gotta get other measurements
3 2 people (me and a buddy) with 1/4 tank (6 gallons)
4 used for pleasure
5 yes deep vee, I will get a measurement on deadrise this evening
6 1988 4.3L v6 mercruiser
7 175 hp alpha 1 need to find out for sure on ratio
8 4400-4800 rpm
9 antiventilation height? gotta get a measurement
10 Mercury quicksilver Laser II 23 pitch stainless steel
11 Hitting 4700 rpm only 39mph (gps)

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

Oriole, You are trimming the drive up slowly at WOT, Right? Till almost the point of cavitation, then down a touch till it grabs fully! That's how it's done to attain highest speed.

Yep, just like I am floating above the water

would a 4 or 5 blade work better for me perhaps?

 

[hwsiii]

Re: prop + more hp = more speed @ same rpm?

Oriole, forget the antiventilation height as you have an IO and that can't be changed. There should be a sticker or plate on the back of the boat that tells you the base weight of the boat and the length, I need the weight to run some numbers for your boat and I also need the gear ratio and deadrise. I also would like to know does the stern sit down in the water deep with no one in the boat, you may be going in the wrong direction with your props having high rake, although normally with that much horsepower on that size boat you want high rake to get less wetted surface area, but you normally wouldn't use a hydrofoil to get on plane quicker and theoretically the hydrofoil holds the bow down by lifting the stern.
You have two totally different props you are using, the 23 pitch LaserII should be about 27 degrees of rake with a 4" progressive pitch, which means that the trailing edge pitch is 25" and the 19" pitch aluminum Black Max has about 16 degrees of rake with regressive pitch, which means the trailing edge is LESS than 19" pitch and is the exact opposite of the Laser. This just goes to show that blade geometry can have MUCH more effect on RPM than pitch because the difference in RPM is only 200 RPM and it should be a minimum of 750 RPM between the same model of prop and a change of 19" pitch to a 23" pitch, although the hydrofoil could be skewing these numbers to some degree.
They both show a minimum 27% Prop Slip no matter what gear ratio you have 1.32 to 1.98 and the aluminum prop has LESS slip than the stainless steel. That is a very high slip ratio for that boat with that kind of horsepower no matter what it weighs and the deadrise of the hull. When you get the gear ratio I can tell you exactly how much prop slip you have.
You need to take the hydrofoil off and see what the speed is, a lot of the time they get the boat up on plane quicker but they slow down the top speed a lot as they can create extra drag especially on an IO that can't be raised, try that please.
Without the correct data everybody can only guess at the problem but 25% prop slip for that boat and motor combination is extremely high.


Prop Pitch Change

Prop Slip

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[Oriole]

Re: prop + more hp = more speed @ same rpm?

wow, awesome info there hwsiii !!!
got around to getting some more numbers, This is how I got my deadrise,... back at the transom from the keel to the chine, I put a angle indicator on there that I use for pinion angles on race cars for the driveshafts, the angle there is 20*, then the chine reverse pitches down 10* for 3.5". At the transom side of the chine, it has a slight pitch down, about 3"
towards the middle of the boat the deadrise is 25*
The beam at the widest point where the bump rail is is 80"
I would say the boat sits real low in the stern, about 20" to the keel, I don't like how the bilge discharge is like right at water level. I will try the boat out without the hydrofoil, it does seem to give off a strange wake now.
I could not find any sticker on the back either on the total backside by the swim platform or inside the engine bay, I did find the serial number on the drive and the valve cover, but they did not match, I wasn't sure where to look for a number for the ratio, so I put the engine at TDC, marked the prop with the boat if full forward gear, and turned the engine 1 full turn, and all I got was exactly 1/2 turn on the prop Just short of 2 turns to get the prop to turn 360* (probably 1.84)
I will have to go and take the boat to the weigh station to get that number.

 

[hwsiii]

Re: prop + more hp = more speed @ same rpm?

If you have a real 1.5:1 gear ratio then the prop slip you are showing is astronomical, as it is over 45%. I have never seen REAL prop slip that high and yet still stay within the recommended WOT RPM range, I find that inconceivable. The motor must be fine as it theoretically turns almost the same RPM with both props, even though there is a 4" pitch difference. That leaves me totally baffled. I find that if your boat weighs about 2,300 pounds, which I believe is the maximum it can weigh with 2 people, 6 gallons of gas and some drinks plus base boat weight of 1,800 pounds, and a gear ratio of 1.5:1 with a 175 HP engine you should be able to do at least 40 MPH with a 16" Pitch Prop, not a 23" Pitch. Check your tachometer, something is NOT right. I am just absolutely baffled and making wild guesses here because it just doesn't make sense, I will tell you that you should be using right at a 16" pitch prop in aluminum for that boat and motor combination if your gear ratio is 1.5:1, my program is always spot on for prop pitch with a single engine but it is not always right on speed because of all the variables involved with hull form and where the weight is placed in the boat as well as other variables. Here are my numbers for your boat with a 1.5:1 gear ratio and a weight of about 2,300 pounds.

I will tell you I have a 16' flats boat with a 40 HP outboard and I get about 30 MPH out of it with an allup weight of approximately 1,800 pounds. In comparison your boat should fly. I am sorry I don't have a better answer.

Boat and Motor Info

Prop Picker

Prop Slip

If you would take the boat out and run it this weekend, with the hydrofoil OFF, and use both props and give me the RPM and speed for both of them from 3,000 RPM up to WOT in 250 RPM increments I am sure we can figure this out and find the problem. I will take those numbers and run motor and prop thrust calculations as well as prop slip and motor power available versus prop power required.
Is there a possibilty you are running the wrong diameter props, it just doesn't make logical sense.

H

 

[Oriole]

Re: prop + more hp = more speed @ same rpm?

Thanks for the help hwsiii

I just got back from taking the boat out just by myself, and running on less than an 1/8th tank, even had the engine cover off, lake was a tad choppy with 8 mph winds from the north, not a good day to test, but I did take off the hydrofoil. I only ran the Laser 23 ss. Did act different, starting to get on plane really let the bow go up, but got on plane fairly quick, I let the engine warm up to its normal 150* and opened it up full throttle, going with the wind motor was running hard at 4500 and I caught 43mph on the gps, coming thru the wind was definitly slower somewhere around mid 30's, didn't really bother running it too hard against the wind.

I think you might have misunderstood the drive ratio, 1 full turn on the prop was just a little short of 2 full engine revolutions, I would have to say it is a 1.84 ratio, which seemed to be standard with the V6 in the 80's

The part # on the Laser is OE QUICKSILVER 48-16548 A41 23P
Which is a 13 3/4" x 23p ss prop

I will get those numbers you need sometime this weekend, Thanks again everyone

 

[hwsiii]

Re: prop + more hp = more speed @ same rpm?

Oriole, the problem is solved if you change to a 21" pitch Laser you will gain your RPM up to 4,850 RPM and you shouldn't lose any speed, which would be better but not required. You lost 200 RPM on the 23" but you gained 4 MPH. Your prop slip went from 30% down to 13% when you got rid of the hydrafoil and 1 less person, and 13% is reasonable. Here are the new numbers that i ran for you to show where you are now. And the gear ratio makes sense at 1.84:1.

Boat and Motor

Prop Picker

Prop Change

Real Prop Slip

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[Tail_Gunner]

Re: prop + more hp = more speed @ same rpm?

The boat this gentleman is refering to is a I/O..The leg run's deep and it is pushing a load. WH has stated in previous threads that the Laser lack's blade area and i too first hand have experienced the lack of grip on a heavy boat.

Mikos is as we speak playing with both a laser and enertia......And he has no light weight bass hull. His result's should be intresting.

 

[hwsiii]

Re: prop + more hp = more speed @ same rpm?

That answers that question TG, I thank you buddy, that will be something I will place in my memory for future reference. Hopefully with my memory I can. LOL

H

 

[jdblya]

Re: prop + more hp = more speed @ same rpm?

I am still going to compare my speedometer next to someone else, but, how would I get my speed up closer to 50mph? bigger prop? which would start bringing rpm down so then I would need more engine power to get that back up right?
+< 24 pitch on a v6 typical though for top speed?

I don't think you would see 50mph with a 2bl 4.3?

 

[MikDee]

Re: prop + more hp = more speed @ same rpm?

The boat this gentleman is refering to is a I/O..The leg run's deep and it is pushing a load. WH has stated in previous threads that the Laser lack's blade area and i too first hand have experienced the lack of grip on a heavy boat.

Mikos is as we speak playing with both a laser and enertia......And he has no light weight bass hull. His result's should be intresting.

Looking at Mikos results, & pictures, the blades on that Laser are kinda on the small side, a Stiletto is probably bigger in blade area, and the Enertia blades are somewhat bigger then both, maybe the Stiletto is a better match here, and for Mikos.

 

[Silvertip]

Re: prop + more hp = more speed @ same rpm?

Hitting 50 on what kind of boat? Let's compare apples to apples. I would also be very leary about other peoples claims. Speed of a boat, like gas mileage on your friends new car is often a stretch. Unless speed is measured accurately, it is just another "maybe".