Difficulty holding at a 25 mph speed

[jack7771]

1986 16 ft. Starcraft​

1986 110 HP Evinrude

Using a 17" pitch prop, the boat has difficulty holding at a 25 mph speed. However, using a 15" prop, that is not a problem.
Any idea why?


Jack​

 

[kenmyfam]

Re: Difficulty holding at a 25 mph speed

Many many reasons. It's just knowing where to start.
Set up is most important. How much motor is in the water ?? how much load is in the boat ?? what RPM are you achieving at WOT ??? is the foam waterlogged ??
That is just the start and also assuming your motor is in good shape with compression, fuel delivery and spark. Cupped prop, regular or whatever.

Give as many details as you can and also some photos of your motor and boat out of the water showing the anti cavitation plate and the stern of your boat would help.
There is plenty of help out here but we all need to know as much as possible to give any realistic advice.

 

[jack7771]

Re: Difficulty holding at a 25 mph speed

I know I've got all that info -- just need to dig it up -- all except the waterlog part. How do I know if the foam gets waterlogged?

 

[tashasdaddy]

Re: Difficulty holding at a 25 mph speed

what is your top speed? how many people in the boat? it this the speed that it wants to come off plane? fiberglass or aluminum boat?

doing some research all the 16 ft Fiberglass boat weight +-1,000lbs
the motor weighs 300, and guess the trailer weight.

you can have to complete rig weighted at a Public Scale.
you then have to calculate hull, motor, fuel, gear, trailer weights. if the scale reading is more than the calculate weight, you are carrying extra weight, as in water in the foam. this can be as much as 500LBS.

 

[kenmyfam]

Re: Difficulty holding at a 25 mph speed

I know I've got all that info -- just need to dig it up -- all except the waterlog part. How do I know if the foam gets waterlogged?

Look how the boat sits in the water or better still weigh it and compare to what the manufactured weight should be. You will have to do some basic math to allow for motor, trailer, gas etc still on board when you weigh it.

 

[tashasdaddy]

Re: Difficulty holding at a 25 mph speed

posting at same time.

 

[Chris1956]

Re: Difficulty holding at a 25 mph speed

Jack, The answer is pretty simple. With the 15" pitch prop, your motor is turning about 400 RPM more than with the 17" pitch prop. At the higher RPM, more power is available from the motor.

 

[steelespike]

Re: Difficulty holding at a 25 mph speed

Check those things.^
First thing I notice is that setup should approaching 40mph or more.Motor should be turning about 5,000 to 5,500.And unless that boat is a load the 17" prop should be easy.You might do a compression check and be sure its running on 4 cylinders.Throttle opening all the way?Check the setup.
Antivent plate (just above the prop) even with the bottom of the boat is a starting point usually can run up to about an inch higher.This alone could add
a couple of mph.Does the motor trim up right and the boat get up on the water?(doesn't plow or lay down too flat)

 

[jack7771]

Re: Difficulty holding at a 25 mph speed

BTW, I want to be able to hold steady at 25 mph for water skiing.​

Okay, here's the info:​

Dry weight of boat is 1000 lbs. Motor is 288 lbs. Then hardware, battery, oil resevoir, gas, etc.​

I have not actually weighed the boat -- I'll have to look into that.​

? New 17" Hustler prop (is cupped)
? Motor position = # 2 spot
? Note: Bimini top is on (down - but this does seem to slow the boat slightly)
? 2 people in boat​

RPM..................................MPH
5000 (wot)......................... 37?
4500..................................33
4250..................................31
4000..................................29?
3700..................................27
3100..................................21​

? New 15" Hustler prop (is cupped)
? Motor position = # 2 spot
? Note: Bimini top is on (down)
? 2 people in boat​

RPM..................................MPH
5450 (wot)......................... 36
3800..................................25 --- can hold steady​

Here are 4 photos:​

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Jack​

 

[Chris1956]

Re: Difficulty holding at a 25 mph speed

Jack, With the 17" prop you are a bit overpropped. The 15" prop is just about optimum.

BTW - Unless you are doing tourneyment skiing, you do not have to be at exactly 25MPH. Actually, I like to adjust the boat speed to the skiiers weight and style. Some skiiers like to skip along the surface. Some like a softer ride.

For example, on the way back to the starting point 1/2 thru the run, when towing my wife on salom, I push the throttle to 36+MPH to really get her to rock and roll! That is when she has warmed up, got the rythum, and is ready to do her best skiing.

 

[Chinewalker]

Re: Difficulty holding at a 25 mph speed

I would have to say something is wrong with the set-up. I have a similar Starcraft boat, 2-foot longer, with a similar motor (90hp, not the 110) and also run a 15-inch prop. I'm peaking with a light load in the 5800 RPM range. It would make sense that a smaller, but similar boat with a larger motor with the same prop should spin higher. Which leads to the possibility that the boat is weighed down by soggy foam...
- Scott

 

[Tacklewasher]

Re: Difficulty holding at a 25 mph speed

I'd suggest raising the motor. It looks low compared to the bottom of your hull.

Take it to the top hole and see if the prop ventilates while turning etc.

You should see an increase in RPM (and speed).

Putting your numbers into a slip calculator shows 6.25% with the 17p and 7.7% with the 15p, both of which are pretty good. But you should be able to get closer to 6000 rpm with the 15.

 

[kenmyfam]

Re: Difficulty holding at a 25 mph speed

I'm still in the get the boat weighed side.

 

[jack7771]

Re: Difficulty holding at a 25 mph speed

Well thanks for all the comments. However, it is hard to see a relationship with holding the speed constant.

I have had the motor raised higher on the transom, and that was 2 mph faster, but I brought it down because of too much cavitation/ventilation with skiing and tubing. I'm not concerned about that 2 mph.

The angle of tilt you see is what's needed to properly pull a slalom skiier out of the water. And yes, the boat will go faster if the motor is kicked out another notch. Again, having the most speed possible is not a concern.

And, of course you can pull skiers at different speeds -- many skiers I pull are amatuers, and a 25 mph speed is good for most skiers. Sure, a good slalom skier can handle 36 mph and a 50' rope on a slalom course; and a trick skier may want it held at 20 mph.

It's just irritating that the motor won't hold the sweet spot with the prop which gives me the best gas milage. It's like why not hold steady at that speed if it can do so at other speeds!?

Now it's true that the engine is not up to par --rings are getting old. And maybe the boat foam is getting waterlogged, I don't know. Getting the boat weighed before and after it's in the water will answer that I suppose, but the 25 mph problem exists right after the boat is launched, so I doubt that has a bearing on the problem. BTW, how long does it take for the waterlogging to dry up?

Perhaps I should just be satisfied using the 15" prop -- it does a great job with a full boat load!​

 

[freddyray21]

Re: Difficulty holding at a 25 mph speed

I think as already stated it's simple. Your rig wants to run a 15 pitch prop.

 

[kenmyfam]

Re: Difficulty holding at a 25 mph speed

Foam will take many many years to dry out. The realistic answer is to rip it out of there and install a new floor.

 

[freddyray21]

Re: Difficulty holding at a 25 mph speed

years ago, but similar boat 17 ft Winner tri hull with 135 hp Johnson and it topped out at 40 on a good day. I don't think given your wot speed you have soaked foam. I think it is just fine and the 15 pitch is what you need.