help getting right prop

[SilverSS07]

Hey guys. Been having issues with my boat and finally got her running good. It's a 1975 Bonito 15' tri hull with a 1973 Merc 500. Sorry but I haven't been able to get her weighed. Took her out today and found that with all 4 of us in the boat it won't get up on plane. Not even close. With me by myself it got up on plane but took a bit. I got a new prop recently so I'm hoping they will exchange it. The one on there is a 13 pitch. Seems a lot of the guys are running between a 10 1/2 to a 12 pitch on the tri hulls. I don't have a tach either but it sure didn't sound like it was getting up to operating range (4800-5500).

 

[steelespike]

Re: help getting right prop

Very likely the 13 is too much as you suspected.
A tinytach will serve you well for this motor and any size and brand of different motor.
For the health of you motor it's very important for you to be able to tell what rpm your motor is operating at both for prop selection and results. Without a tach and gps speed lightly loaded its just a guess.
Its also possible the boat or setup is contributing to your problems.
But I would say that If I had to guess I would start with a 10 1/2 " pitch.

 

[Texasmark]

Re: help getting right prop

Knowing nothing else, your mention pretty much says you have the engine loaded too much; hence too much pitch if nothing else is the problem. 2 strokes are chickens....cowards if you will. They have a breaking point and if you hit it they will turn to mush. That's why most folks recommend running at the upper end of the WOT rpm range when at WOT. This usually results in lively performance throughout the power band under varying loads. Obviously if you cut the throttle the rpms will be less. They love it up there.......at least mine have.

50 hp on that boat is not a lot of hp. Back in the day (early 70's) about the smallest engine you saw on such a boat was a 60 (OMC 3 cyl loopers were most common) and that had to be propped right if you had more than 2 folks in the boat.

Mark

 

[SilverSS07]

Re: help getting right prop

Thanks guys. I was thinking of trying to find a 10 1/2 pitch and going with that first. Top speed really isn't a priority since I didn't get the boat to be a speed demon anyway; but being able to get on plane would be nice . I bought a tach but it didn't work so I've got another one coming.

 

[SilverSS07]

Re: help getting right prop

Traded her in today for an 11p. That's all they had in stock. He told me if 11p was still too much he would order in a 10p and swap them out. If the weather is good this Fri I'll take her out and see what happens

 

[Texasmark]

Re: help getting right prop

Traded her in today for an 11p. That's all they had in stock. He told me if 11p was still too much he would order in a 10p and swap them out. If the weather is good this Fri I'll take her out and see what happens

Keep us posted. Feedback helps to keep our replies accurate.

Mark

 

[SilverSS07]

Re: help getting right prop

Gave her a try today with the 11p. It did ok with 3 of us in there but add another 200lb person and it won't get up on plane. It doesn't plane very quickly even with me in there by myself but I think it's probably due to it only being a 50- the boat is rated for 85. I'm sure it's very possible the boat has water logged foam since it is a 1975 but I really don't want to pull the deck up to find out! Floor and transom are very solid. The engine seems to be running great. Still don't have a tach yet . Where would you guys go from here? Do they make a 9p or would a 10p be better to try next?

 

[IamCanadian]

Re: help getting right prop

IMHO, I think you have more issues than just prop pitch. GPS speed and WOT RPM are the minimum of info to give advice on.

There are many factors that all work together to have a well balanced setup.

Any chance for a pic? from the side and level with the prop just to get a better idea of the setup?

Also if your foam is water logged, and you have 4 ppl in the boat, You may really be overloading the boat and that is never safe!

 

[jestor68]

Re: help getting right prop

Since the dealer is willing to work with you til you find the right prop, the next logical pick would be a 10".

Remember; you have a 1973 motor. Have you done a comp test on the motor? These motors will run fine(smooth) with low comp; just won't develop the power they once did. BTW- "new" comp is 135-140 psi.

That boat will weigh about 1500 lbs with 2 people.

 

[steelespike]

Re: help getting right prop

My 65 500 runs an 11" prop with a all up weight of about 900-1000 lbs. with one aboard.
Boat only dry weight is 375. so you can see a that motor, gear, gas, battery and crew can add up quickly.

 

[SilverSS07]

Re: help getting right prop

Hey guys. Comp is 125 on all cylinders. Seems to be running good since I got the fuel issue fixed. I know it's got good spark on all cylinders. I know it's a lot to ask of the old girl with the weight of 4 people (about 550 lbs) + 12 gals fuel, cooler, battery, safety equipment, etc. For whatever reason it never occured to me to use my phone for a GPS speed lol. I'll get that next time I'm out. I'll take some pics this afternoon. According to the previous owner the boat was redecked a few years ago. I would think if the foam was waterlogged they would have replaced it then but who knows. When I was a kid (mid 80's) my dad had a little 14 or 15 tri hull with a 35 Evinrude on it and that little boat would scoot just fine with all 4 of us on it. I was thinking this one would be the same.

 

[SilverSS07]

Re: help getting right prop

Here's a few pics. Sorry the 2nd one is sideways.

 

[Texasmark]

Re: help getting right prop

On a 9 or 10 I'd say it depends upon whether or not you plan to carry the 4th person very often. It seems pretty obvious that you are lugging the engine and as I said yesterday elsewhere, 2 strokes don't like being lugged and part of the reason possibly that they run about 1 hp per cu in. You have apparently fallen off your torque curve and once you do that added load just aggravates an already dismal situation. Pics look ok as to position of engine and all. I'd go with the 9. You and the engine would be happier and it might carry you at 25 mph which is a reasonable cruising speed. Course when out alone, engine might get a bit high in rpms. A tack would help.
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I just ran over to the BAM prop slip calculator. I think your engine runs a 2:1 gearbox so I used that, and 5000 rpm and for a 9" it said theoretical speed was 21 and a 10 yielded 24. Really low numbers considering planing speed would be around 15-18. BUTTTTTTT toss in some prop slip, which is always there since water is not a solid and you are right at planing speed and the more of your boat there is in the water the worse it is. At those speeds a lot of the boat is in the water.

Doesn't look good man.

Mark

 

[SilverSS07]

Re: help getting right prop

Thanks for the help. I'll keep an eye out for a good price on a bigger engine or might put her up for sale and see if I get any bites. Honestly I like the boat and have done a good amount of work to it and don't really want to get rid of it, but bigger is always better . Everyone always wants to go whenever I take it out so going by myself would probably be a very rare occasion. I did some reading on here about the hydrofoil fins but didn't really see anything difinitive other than they can take away from top speed. Think it would be worth a shot?

 

[Texasmark]

Re: help getting right prop

Thanks for the help. I'll keep an eye out for a good price on a bigger engine or might put her up for sale and see if I get any bites. Honestly I like the boat and have done a good amount of work to it and don't really want to get rid of it, but bigger is always better . Everyone always wants to go whenever I take it out so going by myself would probably be a very rare occasion. I did some reading on here about the hydrofoil fins but didn't really see anything difinitive other than they can take away from top speed. Think it would be worth a shot?

The prop slip calculator info I posted shows the speed the boat would achieve if the prop were spinning in a solid; like a nut running down a screw....no slip. You cannot move the nut up or down the nut at a given rotational speed faster than the thread pitch allows (the blade pitch of the prop) regardless of the load.....within reason of course.

To move the boat faster, and I believe you are right at marginal performace as we discuss this, requires one of two things before you even begin to talk about prop slip which could get as high as a 25% speed loss under heavy loads:

Rpms have to increase or pitch has to increase. To do either in your apparent situation requires more "hosses" as you said.

HTH,
Mark