what is limit for 15 ft fiberglass boat

[bjromaine]

I have a newly acquired 15 ft fiberglass boat that is tri hull. It is rather old but in good condition.
It is powered by a 70HP Evinrude which is also old. Pre 1990's.
HOw many people should be able to ride in boat and still get on plane?
How many people should be able to ride in boat and still get on plane with a 2 person tube in tow?

Approximate speed expected with 2 - 3 people?
What is WOT of this motor?

Haven't been boating in decades so need updated information.
Remember skiing with small boat and motor but can't remember limitations.
Just tried to pull 2 adults in tube with 2 in boat and could not get the boat planing.

Any suggestions greatly appreciated.

 

[Watermann]

Re: what is limit for 15 ft fiberglass boat

No way to know for sure the exact answer to your questions (people all weigh differently) but it seems your out on the lake answering them yourself. The operating range of your motor is probably 5000 - 5500.

 

[boobie]

Re: what is limit for 15 ft fiberglass boat

Take it out and experiment with it.......

 

[Chris1956]

Re: what is limit for 15 ft fiberglass boat

That boat motor combo should be a pretty good match, as 70-75HP is probably near max for that boat. It should plane with 4 adults, quite well is prop is correct and motor makes normal power.

Tubes have lots of drag. You should be able to pull one adult in tube with 3 in boat, I would think.

 

[V153]

Re: what is limit for 15 ft fiberglass boat

That boat motor combo should be a pretty good match, as 70-75HP is probably near max for that boat

Ehh, but not for pulling toys. Think our boy's gonna need a lil more hp for that.

 

[bjromaine]

Re: what is limit for 15 ft fiberglass boat

Thanks for all the input.
I wasn't sure what was normal but was achieving about what everyone thought would be normal.
With choppy waters, 3 in the boat and 1 large person on tube, I could only get on plane after adjusting the trim (manually) out but that caused very short intermittent ventilation problems.

I need to put a tach on the motor and I think it may be a bit low....have not checked the prop pitch and size yet nor have I looked to see if the throttle cable is getting full throttle.

Will be looking into that soon.

 

[metriccrescentwrench]

Re: what is limit for 15 ft fiberglass boat

... I could only get on plane after adjusting the trim (manually) out but that caused very short intermittent ventilation problems.

Your engine doesn't have hydraulic trim? I'd think that 70 HP would do pretty good on a 15 ft boat, but without trim it would be kind of a drag.

Look for the model number of your engine on the transom bracket, from that the year can be determined.

 

[sutor623]

Re: what is limit for 15 ft fiberglass boat

That Evinrude isn't old!! Just barely broken in at that age. Those 70hp motors are designed to tach between 55-5800 I believe.

 

[boobie]

Re: what is limit for 15 ft fiberglass boat

Correct............

 

[bjromaine]

Re: what is limit for 15 ft fiberglass boat

This is a 1980 motor. I finally tached it and the WOT is only 4600. Now it is a question is the prop pitch too great (13X19) or is the timing off. The carbs are synced and fully opened. The only info I have regarding the timing is a picture of the linkage adjustment that I found online. Seems it should be 4 31/32" but is only 4 28/32".
This can negatively affect the WOT correct?

 

[kjdunne]

Re: what is limit for 15 ft fiberglass boat

Timing can affect the performance. Always heard it speced in degrees, don't know about linkage adjustment in inches. You should do a link and sync per the factory manual for that engine year and HP. Do you know when the carbs were last cleaned and rebuilt? Clogged jets are a more common WOT problem. The prop pitch may be a little high, but you need to get the engine running right before you can really tell.

 

[82rude]

Re: what is limit for 15 ft fiberglass boat

hold on there pardner!your over propped .for what you want to do a 15p is more likely.

 

[bjromaine]

Re: what is limit for 15 ft fiberglass boat

I agree with both KJ and 82 but think the timing should be done first or the prop fit won't be known for sure. I was hoping to find the specs on the linkage without having to dole out the $$$ for the manual. The timing is supposed to be 19 BDTC at full rpm's. That is controlled by the linkage I mentioned.
I think I will try the linkage adjustment first then if the motor runs fine, lower the pitch. Not sure whether to go with a 15 or 17 pitch as I have read several threads that both were claimed to be the right pitch for a similar setup.
Thanks for the input.

 

[sutor623]

Re: what is limit for 15 ft fiberglass boat

Clogged jets are a more common WOT problem.

Depends on which jets are clogged. Typically the slow speed/idle jets are the first to show symptoms in these motors.

Most people do not mess with the WOT timing on motors. If anything, the throttle linkage could be preventing the carb butterflies from opening all the way. I would just check to make sure (on land) that when you peg the throttle, the carbs open fully. In my experience, dirty carbs presented idle issues, and erratic WOT. I doubt that is the issue. You are definitely over propped, no question. I would say that with that much weight on the boat, you would want a 15p, no question. If you plan on cruising solo, or with one other, then a 17 would be a better match. I just bought a turning point hustler, in a 17P. It has exhaust vent holes, so it should have the hole shot of a 15P, and the top end advantages of a 17P. Relatively similar setup as you, 16' aluminum starcraft with '84 75HP Rude.

 

[bonz_d]

Re: what is limit for 15 ft fiberglass boat

With no one in the boat does it sit high or low in the water? I ask because many of those old boats look great yet under the floor there is floatation foam that over the years becomes water logged which adds more weight than you would believe.