Yamaha 15hp - boat not getting on plane

[bega]

I was hoping to get some advice on getting my boat back to optimum performance. I currently own a 3.66m aluminium v nose punt and it is powered by a 15hp Yamaha 2 stroke short shaft outboard. I have fitted a floor and front and rear casting decks and have found that it now struggles with another bloke on board to get up on the plane. It is fine with just myself but once I add another person to the boat the motor struggles to lift the boat onto a plane. I have tried adjusting the trim but to no avail. The decking was 19mm form ply and I used about 3 sheets (1200x2400) so there is a fair bit of weight just in the flooring and decks. A sheet probably weighs 25kg so all up about about 75 -80kgs. <br /> <br />How much weight can a boat of this size handle and from memory I think the boat was only rated to 15hp maximum? Can I upgrade to a 25hp without making it too unstable once up on the plane? If so, should I buy a long shaft or short shaft? If I buy a long shaft I will probably have to add more height to the transom where it is cut away?<br /> <br />Thanks and I appreciate any advice you can give me on this matter. I just want to be able to take one of my mates fishing every now and then and not worry about the boat not getting on the plane.<br /><br />Thanks<br />Jason

 

[JB]

Re: Yamaha 15hp - boat not getting on plane

Howdy, Bega.<br /><br />Welcome to iboats. <br /><br />There are a lot of possible things you can do, but they are not engine problems, they are boat rigging and prop problems.<br /><br />I am going to transfer you to the Boat Forum.<br /><br />Good luck.

 

[Boatist]

Re: Yamaha 15hp - boat not getting on plane

My small aluminum boat is 14 feet or 4.26 Meters with a 15 HP Johnson and it has the same trouble.<br />Wife and Me and It gets up fine, another buddy and fishing gear and it will not plane.<br /><br />You can try moving batteries and gas tanks around and see if it helps. Likely you will need to reduce weight or get a bigger motor.

 

[WillyBWright]

Re: Yamaha 15hp - boat not getting on plane

Lets see, worked great ... added floor and other stuff that adds significant weight ... now not working so geat. Hmmmmmmmm

 

[Bondo]

Re: Yamaha 15hp - boat not getting on plane

Originally posted by WillyBWright:<br /> Lets see, worked great ... added floor and other stuff that adds significant weight ... now not working so geat. Hmmmmmmmm

Ayuh,........<br /><br />I sit here and read about all these fellas who are building Bassboats out of Little Bitty Rowboats,+ Jonboats.............<br /><br />I figured it's only a matter of Time before someone would figure out that Payload is Payload,........<br />Whether the Weight is Used Up with Rigging,... Or People,... Or Gear,........<br />There's Only So Much Weight you Can Float,.... With a Little Bitty Boat............ <br /><br /><br />BTW,........... More Horsepower is NOT the Answer.................

 

[Texasmark]

Re: Yamaha 15hp - boat not getting on plane

And 15 hp is not much hp. Having grown up with a 10hp Scott Atwater on rented boats with a 250# dad for a fishing partner I can surely attest to that.<br /><br />But the BIA certifies boats for what they consider safe hp. Get above that and you put something at risk.....I guess. <br /><br />Haven't read their guidelines but I do know part is based upon beam and length......which is sorta a joke.<br /><br />My current boat is a high rake 17' aluminum with an 84" beam. The nose comes to an abrupt point so the first couple feet of the boat don't really exist.<br /><br />The bottom of the boat is 5'(60"). However the Gunwale is 84", thanks to the sides flaring out and a couple of strategically placed pieces of extruded aluminum. Empty the boat weighs 700#. It's BIA certified for 130 hp. A 90 drives it right at 50....better be hanging on. Can't imagine what a 130 would do.....but it's legal!!!<br /><br />So is more hp your answer? Dunno.<br /><br />Mark

 

[WillyBWright]

Re: Yamaha 15hp - boat not getting on plane

I picture that as a 12' jon boat. They don't tolerate added weight very well. If it were a 15', that added weight would have less afffect. Don't mix longshaft motor on shortshaft boat. Heavier motor plus adapter weight...double whammy. A 12 footer is probably rated for max 15Hp. Stay with that. Adjust the weight factor. Sometimes you can use thin plywood with better bracing in the stress areas to reduce weight. Have a look at the capacity plate and realize that the weight you added would be part of the maximum capacity for passengers and gear.

 

[cobra 3.0]

Re: Yamaha 15hp - boat not getting on plane

You may try a shorter pitch prop. If it's a 13" try an 11 or 10. The added weight is just like having an extra big buddy on board all the time. A 15 can easily handle three people and plane.

 

[bega]

Re: Yamaha 15hp - boat not getting on plane

Thanks for the advice fellas. Maybe more Hp is not the answer - maybe I should be happy with the fact that it can still get up on the plane with just me in it so SOLO trips is it until I upgrade to a bigger boat. I just have to convince the boss of the house that for safety reasons I need a bigger boat .<br /><br />Has anyone used different props to get more power - how does that actually work I assume there are pros and cons to everything and there would have to be some cons to putting on a bigger prop???<br /><br />What about adding more floatation, would that assist with the boat being able to get on the plane???<br /><br />Anyway I think I better go back to the drawing board.

 

[Bondo]

Re: Yamaha 15hp - boat not getting on plane

The Prop Pitch is like a transmission,.....<br />You'd need a Smaller pitch for a Lower gear, to Make More Power,..... But it would be Slower......<br /><br />Adding Floatation, Adds Weight,+ will Slow you down Even More.........<br />Floatation Doesn't Float a Thing,.... Til After the boat Sinks.........<br /><br />The Only way to Float More Weight,..... Is to Displace More Water.............<br /><br />Enjoy what you Have,... Til you can get a Bigger Boat............

 

[mjbrueck]

Re: Yamaha 15hp - boat not getting on plane

On small engines, you don't have a whole lot of pitch to play around with (the factory prop was probably around an 8-10" pitch). You might find that if you drop down an inch in pitch that you're hitting max rpms before getting on plane. It would be easier to regain the speed by dropping the weight.<br /><br />If you do keep the current setup, don't lug the engine when there's two of you in the boat. Run it at a fast idle.

 

[bega]

Re: Yamaha 15hp - boat not getting on plane

I might put on one of those hydrofoil fins - they reckon they can aide in getting boats on the plane and keeping the boat stable whilst on the plane? What do you guys think????

 

[WillyBWright]

Re: Yamaha 15hp - boat not getting on plane

Might be worth a try. I find that they do more to slow the boat down than lift it up, but...

 

[steelespike]

Re: Yamaha 15hp - boat not getting on plane

Your main problem is you need a boat with a bigger footprint. Try lightening the load.Use a 3 gallon gas tank instead of a 6.If you are carrying a cooler try a 40 qt instead of a 80 qt.Try a light fluted anchor instead of a heavier one.light plastic oars instead of the wooden ones.place weight very carefully.<br />I have had good luck with the doelfin at the price you could give it a try.<br /> You could also try raising the motor virtically.<br /> raise the motor 1/4 inch at a time until just before ventilation becomes a problem use 1/4" sticks to set the height.Just be sure the motor cools properly and the clamp has a good grip.Probably won't help much but give it a try.It may at least improve the light load performance.

 

[Luna Sea]

Re: Yamaha 15hp - boat not getting on plane

Start drilling holes in the floor with a holesaw. This will lighten it up.

 

[Boatist]

Re: Yamaha 15hp - boat not getting on plane

As I said in earlier post I have the same problem with my 4.26 meter aluminum boat with a Johnson 15HP. My motor came standard with a 3 blade 9 1/2 diameter by 10 pitch prop. Again it work great if just the wife and I went for a ride, but if I went with a buddy over 200 and fishing gear would not plane.<br /><br />I bought a new Stainless steel prop 9 3/8 diameter by 9 pitch by 3 blade. This prop did push the boat faster but still would not make it up on plane. Also this prop did not back up as well as the orignal.<br /><br />I never tried a lower pitch prop but a 8 pitch may have been enough to push it up on plane.<br /><br />This boat and motor did not have a Tach so I did not want to take a chance on over reving the motor.

 

[Jdeagro]

Re: Yamaha 15hp - boat not getting on plane

Your best Solution is Our Smart Tabs ST780-20. WE started out selling Smart Tabs for 10 to 12' boats. primarily tenders. The results will be even better than a 25 HP motor.

 

[Manduchi]

Re: Yamaha 15hp - boat not getting on plane

Its not a horse power problem, Its a weight problem.<br /><br /><br /> Your boat can probably haul around 200-250 KG.<br /><br /> You threw in 80 KG worth of flooring????<br /><br /> By the time you and your freind jump in with all of your supplies...... <br /><br /> Your over loading your boat. <br /><br /><br /> Sure , if you throw enough horse power behind it any thing will come up on plane. Its not going to matter much if it takes on water and sinks though.<br /><br /> I wouldnt upgrade your motor, I would upgrade your entire boat.<br /><br /> A hydro-foil isnt going to make any difference. Foils wont give you extra horsepower to lift your boat out of the water.<br /><br /> If it cant plane now, it wont plane with a foil.