Can this do better

[mike1117]

I want to get top end speed faster and know zip about props. I have a fiberglass 1988 Four Winn's 18' horizon, open bow with a 4.3L cobra engine and a aluminum 3 blade 15x17 Michigan prop. ( I don't know what 15x17 means) I've had the boat for 10 years, no engine problems and very clean. Usually it's me and my son or wife and daughter too, total about 600 pounds of people. It seems to max out at about 4200 rpm's with a max speed of 32 - 33 mph using a GPS. It will run up to 4800 rpm's, 4600+ is the red zone; but no more speed. rides best with kids about 250 lbs in the bow. It is stern heavy with that 4.3L. With me alone 260 pounds seems like I can get 35 mph on flat, calm evening water. I was thinking I should be able to get at least 45 mph; Mostly run on NY finger lakes or lake Ontario. I read through the cobra engine/prop on this site, but no insights for me.
Thanks,
Mike:redface:

 

[tmh]

Re: Can this do better

Finger lakes and lake Ontario .......hmmmmm........maybe it will go faster once the ice melts! Mine tops out at 0 mph right now.

OK, seriously, lots of factors .....but assuming your tach is correct and the engine runs fine and the boat bottom is clean - you may get a bit more with a 19" (pitch) 3 blade SS prop. Maybe add 5 mph and bring your max rpm to 4300 range or so.

 

[steelespike]

Re: Can this do better

I'm not an expertt but::
15 X 17 (diameter X pitch) pitch is the important demension. Diameter follows the design based on the application.Diameter usually not considered except certain specific conditions.I'm guessing that with the right prop you should be around at least 40+ mph.Unless the Four Winns is really heavy I woulkd think it would run a 21 or 23 pitch for top end.But this doesn't add up with your present rpm As tmh suggests a 19 should put you at 4200 to 4300.
You might verify your tach is accurate perhaps with a borrowed shop tach.
Is your prop in excellent shape?Are you trimming in (down) to accelerate and out (up)for best speed?Is the motor in excellent tune, good compression?
Throttle opening all the way?
If the present prop is aluminum you might consider a stainless prop or a Turning Point Rascal prop in aluminum.But I would check the other points I suggested first.

 

[mike1117]

Re: Can this do better

Thanks for the feedback. I don't know if the tachometer is right, I am pretty sure the speedometer has personality issues as does the through the hull depth sounder. Compression, I should have that checked. The boat always hum's nicely and after 7 years of learning to trim it the last three have been a real joy on the water. The prop was fix last year after running up on a shoal, but I've always has cavitation on the prop on the end of the ears the paint on two props has worn off in the first three or four hours of use ( no shoals). Motor gets a tune up every other year, and I get it winterized and setup for spring every year by the same shop. I try not to abuse it, Yeah I trim it all the way down and it pops out of the water onto plane in 3 or four seconds, then I trim back and throttle down to about 3100 rpm and can cruise at about 21 mph.

Thanks for the help

 

[stackz]

Re: Can this do better

almost makes me think your tach is off.

I dont know if marine 4.3 engines are built differently than automotive 4.3 engines but an auto 4.3 has a redline of about 6,000/6,500 rpm.

I highly doubt they use substandard rods/bearings/valvesprings/etc in marine applications so unless your boat is just plain too heavy, the engine is starving for fuel, or its a less powerful engine...you should be able to get more rpm wise.

maybe think about building the engine up for a little more power? though, for all I know, the drive system may be the limiting factor

 

[Silvertip]

Re: Can this do better

A marine 4.3 does not have anywhere near a 6000 - 6500 rpm redline. 4800 is at the top of the range. Marine blocks and most innards are the same as a automotive 4.3 but the cam is more of a truck grind. The 17P (17 inches of pitch) sounds like it is blowing out (losing its bite) or you are trimming out too far. I agree that you should be in the 40+ range and that the 17P prop is not right but until you do more testing, understanding how trim affects this situation, and verifying the tach and speedometer you should not even think about a new prop.

 

[blouderback]

Re: Can this do better

I'm thinking that the prop's hub may be a little "spun", that is, slipping at higher revs. It's odd that your speed doesn't seem to increase from 4200 to 4800 RPM. One way to test this is to put a reference mark across the prop and the hub so that they line up, then run the boat at high rpm, then check your lines. The two lines should not move away from each other.

You might try a stainless steel prop, with a couple inches more of pitch, like a 15 x 19".

Also, hearing you say that the boat is stern heavy, tabs (either automatically adjusting Smart Tabs(tm) or regular adjustable trim tabs like Bennett or Lenco) will also help get the stern out of the water and up on plane faster, and allow you to hold plane at lower speeds. You should also see an increase in top end speed, as the tabs will bring more of the stern out of the water (less resistance).

 

[haskindm]

Re: Can this do better

You should also see an increase in top end speed, as the tabs will bring more of the stern out of the water (less resistance).

Tabs cannot bring more of the boat out of the water. All they can do is raise the stern and lower the bow, actually putting more boat in the water. At top speed the tabs should be all the way up: once they are up they have no effect on the boat. They are great for lowering the bow, which will allow it to cut through rough water and get on plane faster, but they cannot do much to help top speed, only prop selection and trim can do that. Sounds like he may need a little more pitch to get his RPM back down to the redline setting which MAY get an extra MPH or two and will only hurt his "hole shot" a little. I think gaining another 10 mph is unrealistic with the numbers that he has given.

 

[blouderback]

Re: Can this do better

Tabs cannot bring more of the boat out of the water. All they can do is raise the stern and lower the bow, actually putting more boat in the water. At top speed the tabs should be all the way up: once they are up they have no effect on the boat. They are great for lowering the bow, which will allow it to cut through rough water and get on plane faster, but they cannot do much to help top speed, only prop selection and trim can do that. Sounds like he may need a little more pitch to get his RPM back down to the redline setting which MAY get an extra MPH or two and will only hurt his "hole shot" a little. I think gaining another 10 mph is unrealistic with the numbers that he has given.

According to John from Nauticus (the manufacturer of Smart Tabs), as well as several independent testimonials from the likes of Powerboat Magazine, Boating World, Boating Magazine, and Propeller Magazine, these tabs will, in fact, improve top end speed. http://www.nauticusinc.com/testimonials.htm.

 

[mike1117]

Re: Can this do better

You are All great resources for assistance, let me tell you how my boat runs on a typical 1 to 2 ft chop. the finger lakes are almost never flat water and the chop is something I have to deal with all the time. On lake Ontario, it's an inland sea, which sometimes translates into white caps of 4 to 6 feet or long swells of 4 or 5 feet depending on the wind. Either way, I stay at the dock and drink and eventually fall down. It's safer and people will at least find my body.

On a 1 foot chop I can get the boat on plane and run it at 1/4 trim. As I move to 1/2 trim I will start to "porpoise". If I'm alone, I'll deal with a bit of bounce, cause that's what guys do. until one of the tackle boxes pops open. If I have someone in the boat with me I can move then into the bow and stop the porpoising. Here I'm getting about 24-26 mph at about 3500 RPM's.

With my hull in a two foot chop, If I go much beyond 3100 rpm's trimmed so I'm on plane the trim meter is somewhere between 0 and 1/4 and I get about 17 or 18 mph as measured by GPS. Faster, than that gets 2500 pounds of boat an people airborne. Pretty to watch, painful to experience.

When I said stern heavy (stern is the back right) I meant that the boat sits in the water real low at the dock, the bilge out flow port is about 2" above the water line. I swear that some days on rough water where I've got the boat trimmed so the bow is riding high so as not to get waves over the bow, that water is coming in through the bilge hose, seems that on those days I dump about 2 or 3 gallons of water out of the boat when I take out the but plug.

I trailer the boat everywhere. Looks like in another month when the ice melts, I'll give some of these suggestions a try. I like that check the marks on the hub and prop for slipping. The engine always seems to run so smooth, never rough, though It does cavitate quite a bit between 500 and 2000 rpms no matter how I trim it.

 

[Tail_Gunner]

Re: Can this do better

You have some type of prop issue...You said youve had it rebuilt...If i knew the drive type it could be helpful..there's more than a few of those...and im lost..

You should be able to turn a 18p pitch 3 blade alum quite easily..Id start there. what makes no sense is the total loss of prop above 3200 yet you can climb out of the hole with a load...

If I go much beyond 3100 rpm's trimmed so I'm on plane the trim meter is somewhere between 0 and 1/4 and I get about 17 or 18 mph as measured by GPS. Faster, than that

Geesh 3100 you should be hittn 26-30...You either have a motor on it last leg....or a prop with no legs..

 

[Randybeall]

Re: Can this do better

Sounds like you need a good 4 blade stainless prop designed to give you lift at the stern. Best guess would be an 18 pitch. You need more bite in the water and the ability to lift the back of the boat with the prop. Four blade will smooth out the loads of the chop by keeping more blade in contact with the water at all times.

 

[Tail_Gunner]

Re: Can this do better

Good thoughts but something else is very a-miss here. Lets solve that first then go to ride quality..

 

[Randybeall]

Re: Can this do better

I am in complete agreement. That statement of 3 seconds to plane sounds like a lot of power. Perhaps there is a problem with exhaust getting into the prop at the lower unit prop interface. A close inspection of that area would be in order. My thoughts on the 4 blade are in reference to the possibility of engine overpowering the prop currently on the boat. It is loosing the ability to accelerate further right at torque peak, like a tire starting to spin on a car when 'the cam comes on'. Perhaps another thought would be look carefully at everything on the boat, is there anything you can move forward in the boat. A little better balance could help ease the beating your taking.