Small race boat with 1986 15hp motor need upgrade prop

"Hi guy's , it's Dino

"Hi guy's , it's Dino from Australia. I have recently restored a late 80's fiberglass miniture Ski boat . I don't know the name of it but it looks like the big thing only smaller as it's only 2.5 meters long. I purchased a Mercury outboard for this boat and the seller told me it was around the 1986 model and 15hp. The first time I took it out it feels it needs more power . I have seen these types of boats raced by younger kids in the 15hp class and they flew on the water but not this one. With full throttle it feels like it should be going faster. The motor is in top condition near new as it was hardley used. I'm thinking is there a race style of prop I can fit to this motor and of what type to give it more speed? Maybe there is a race kit for Carburator also? Motor serial is OC131711, Thank you for your time . Regards Dino. P.S I forgot to mention that the motor came with one of those hydrofoils fins ( SE sport 200 ) which is still fitted."

"Have a look at solas probably

"Have a look at solas probably a 10, or 11 inch stainless.They are a good little motor you wont gain much from engine mods.prop and engine height ,remember to watch water pressure."

"Thank you David , I'll ch

"Thank you David , I'll check them out. Regards"

"Yes, David's suggestion i

"Yes, David's suggestion is excellent. I run an 11 pitch on my 15 horse (when I'm loaded light - total weight about 600 pounds) and the results are quite good.

If you don't want to go the cost of stainless you can get a Solas Aluminum composite, cupped blade (Alcup 3) for less than 50 bucks including the hub kit - very nice prop.

And your motor is "newer" than 86 for sure. Merc didn't make that model of 15 horse until 88 and your serial suggests it was built during the 1989 production run.

This motor is already putting out about as much horsepower as it can without really stressing it, so keeping it well tuned and running a decent prop is about all you can do to get the best performance out of it..."

"Thanks for the info Graham, I

"Thanks for the info Graham, I went onto Solas web site and seeing my boat is a two seater and probably weighs 700 to 800 pounds with people in it, do you think I will need the 10 or 11 pitch prop as I need more speed top end ( take off speed does not concern me as it's not being raced). Regards Dino"

Just a bit more info. I rang t

Just a bit more info. I rang this moring Solas and spoke to the owner. He said that my old prop is a 10.5 pitch and the best for my application would be a 10 pitch stainless. He said that I should position the motor as high as possible to get more preformance. Would this be ok? regards Dino

"Yep ,Just watch the water pre

"Yep ,Just watch the water pressure.Make sure the teltale is constant"

I find it interesting when eve

I find it interesting when everyone starts to suggest prop pitch and there is no mention of RPM. One guy says you won't gain much from engine height and another guy says you should position the motor as high as you can. And whats this thing about water pressure? Is there a water pressure gage on this motor and if so what should it read?

"Allan, normally I would agree

"Allan, normally I would agree with you, but in this case we are talking about a very small boat with a "low" horsepower motor.

The WOT range of the 15 horse is 5000-6000 rpms - a huge range. The largest pitch commercially available is an 11 pitch, with 9 being the factory equipped "normal" pitch.

So in almost any conceivable situation you can almost use whatever pitch you want unless you have "way too much boat for the motor".

But, on a higher horsepower motor, which usually has a tight WOT range (say 5000-5400) I absolutely agree that pitch AND diameter selection must be made to optimize rpms.

Ditto motor position/trim etc etc etc - but ultimately, in MOST cases, any performance gains will be minimal.

After all, we are talking about pleasure boats here. I'm sure it's important that a racer get every last ounce of performance out of their boat, but for the average boater, most times minor little "tweaks" will provide no "noticable" performance increase.

In the end a properly pitched motor, that is kept tuned up on a boat carrying a balanced load (that is as light as possible - so take out un-necessary equipment/junk) will get the best "gas milage" and maximum performance.

All the other enhancements advertised to do all kinds of great things for your boat usually do little more than lighten your wallet..."

"Graham,
No qualms about your

"Graham,
No qualms about your statement at all, but what was Dino’s question?

“The first time I took it out it feels it needs more power . I have seen these types of boats raced by younger kids in the 15hp class and they flew on the water but not this one. With full throttle it feels like it should be going faster. The motor is in top condition near new as it was hardley used. I'm thinking is there a race style of prop I can fit to this motor and of what type to give it more speed?”

So, just for the sake of argument lets say Dino’s motor is only turning 4000 RPM WOT. WE don’t know. But if that were the case It would be easy to suggest a smaller prop to increase his performance. Am I wrong? And lets not forget this:

“I have recently restored a late 80's fiberglass miniture Ski boat .”

He has restored a fiberglass boat. Fiberglass being the key word here. We’re not talking aluminum. For all we know this thing is heavy as ----."

"Absolutely agree with you - w

"Absolutely agree with you - we don't know what rpms it's turning or how much weight it's pushing.

A 15 horse should be able to push along about 800 pounds of boat/equip/people etc at about 20 mph.

Drop the weight being pushed to 400 and could see 25 mph.

Take it down to 250 pounds on a hull designed for speed and you would rip along in the mid 30's.

Having said all that, his statement is rather subjective. The boat may be zipping along at maximum but to "him" it seems sluggish. He "feels" it should be going faster - but how fast is it going? 15 mph which may be all she can give due to weight/hull design?

Playing around with a prop on the "average" boat that has a 15 hooked on the back may squeeze you 1 mph here or there, but that's about it.

A 4 blade will be slower than a 3 blade, but will give you a better hole shot and quicker plane.

A 2 blade will give you better speed than a 3, but only if measured with a very fine instrument - seriously if a 3 blade gives you say 17.4 mph dropping to a 2 blade (on this motor) may put that up to 17.8 - not exactly something you are going to "notice" or "feel" - although you would feel the additional vibration since the a 3 blade runs smoother than a 2.

I guess my whole point is, the motor and prop should be fitted to the rig it's hangin' on and you have to accept whatever result it will produce for you. If you want MORE SPEED you either change the motor to something bigger or swap the boat for something designed for speed.

There is little to nothing that can be done with the "average rig" to significantly improve performance if all components are properly matched in the first place..."

""There is little to nothi

""There is little to nothing that can be done with the "average rig" to significantly improve performance if all components are properly matched in the first place..."

That about says it all "

"It really does. It comes down

"It really does. It comes down to physics.

If you have a 14 foot boat, that has an actual displacement of 1000 pounds and you want to go 30 mph, then you need a 35 horse motor.

And no matter what you do to a little 15 horse - unless you hang a second one right beside it, it's not going to get you even close to the desired speed.

Performance oriented boaters should really do things backwards - decide how fast they want to go FIRST, then select a hull condusive to that and then calculate the "required horsepower" to achieve their expectations.

Would save alot of playing around and alot of disappointment at a later date... "

"Thanks for all your inputs gu

"Thanks for all your inputs guys, I have taken them all into consideration. I think my main problem is that the short shaft motor is sitting too low in the water as when I took it for it's first run I did not see the hydrofoil come out of the water on full throttle. I only thought about this because I just came across an article on the web saying that the hydrofoil should be just out of the water to achieve maximun speed. So when the prop arrives hopefully tomorrow I'll reposition the motor and take it for another test on the weekend. Regards Dino Boat

"A picture tells a thousand wo

"A picture tells a thousand words .can you take a pic of the cav plate in relation to the bottom of the boat.Now here is a guess, you need a 25-30hp,I would go Tohatsu over Merc based on the stronger/engine mounting system ,but that is just me .I mean to say engine mounts.Your boat is quite a bit different to what the kids race in the 15 hp class.I was expecting a different boat to what you have .Where you in Aus.??"

"Yes David I was thinking of g

"Yes David I was thinking of going bigger in engine size but I'm worried that it might sit dangerously too low to water line as the 15hp now comes very close. I also have the fuel tank at the back with no room at the front to shift it.
I'm in Melbourne Victoria. Regards Dino.
Baot pic

"Yes, the one nice thing about

"Yes, the one nice thing about the 2 stroke 15 horse is it barely weighs 75 pounds.

Any way you could move your gas tank up under the foredeck - that would shift about 6 pounds per gallon up into the nose if balance is a bit of an issue.."

"No room under the foredeck Gr

"No room under the foredeck Graham as there is a fiber glassed storage box which goes all the way . It is acessed from the top of the boat through a hinged metal hatch and holds life jackets, anchor and so on"

From that pic it looks as if t

From that pic it looks as if the motor is still quite low in the water .You should be able to safely raise it and see how you go .

"Hi there,


My tuppence wo

"Hi there,


My tuppence worth...

I've seen and heard of "new" motors with issues. Have you checked the compression on the motor?

Secondly, what are you comparing against here? Do you have a GPS to check the top speed?

Also, as has been noted, are you really getting full RPM @ WOT? My rig had an issue with the control box not opening the carbs fully. I had to trim the plastic sleeve on the motor side of the control to achieve WOT. Jumped from 4.2k to 5.5k immediately. Try find a digital RPM device, works a charm on the water.


The chainsaw guys, STIHL, has a great one that I use. Check their website for one.


Cheers from a hot & sunny Johannesburg!"