What size outboard

You'd think it woulda, coulda, and shoulda planed....if it was putting out the 18 hp.

 

Thanks for your interest,I was aware of the new 2 stroke rules,The boat is from the 70's/80's
I think,so no plate.

 

I suspect the weight to prop ratio with the old prop was not quite getting there,
I think the prop was stopping the engine from reaching it's max revs,
hence the huge wake,and unable to plane.
I think with the new 9X9 prop it will reach it's WOT.
It only reached about 8mph,and when I tried my 5hp Yamaha emergency engine,it did the same, 8mph.
Seeing as I have spent a lot of time on the setup,I would like to sort out the problem,
even if I do get another engine.
I must say,it's been an interesting learning curve,Haha.

 

The new picture you sent shows that the boat has a wide totally flat section in the keel. It really should plane easily with that hull form. It also shows the anti-ventilation plate below the keel line, but not as badly as it looked from the other angle.

Probably your Evinrude is not working that well anymore. Same speed, only 7 knots, as with 5 hp and not lifting the bow up tells about not much of thrust (or an extremely heavy boat). What rpm are you getting?

 

I haven't been able to check the revs with my tacho yet,I'm 40 miles from the nearest water,
it's winter and I'm no spring chicken.So I will have to wait for the better weather.
In my test tank the engine easily reaches top revs or beyond,if I put it in gear and give it full revs,it empties the tank in seconds.I think the engine came off a displacement river cruiser,
(river Trent) hence the 9.25X11 prop.
I don't think the boat gains enough speed to create a wake trough,so the engine can't reach
higher revs because of the torque,I think it will with the 9X9 prop on it.It displaces a lot of water,and the wake is huge but wide.At first it gets going quite well,but after half throttle to full throttle it gains very little speedwise.(Like being in too high a gear in a car)
These engines are still very popular in the States,I've done a bit of research on them.
I'm pretty confident the engine is running correctly.

 

I had a similar size boat and tried the same motor on it, my results were the same as yours, it was just about to plane, but couldn't quite get there.

That size boat typically had a 40-50 hp rating, 15' glass boats were normally in the 90HP max range. (I had both).

An aluminum boat that size might work well with an 18, but on a glass boat with the right prop, it "may" get on plane.

My 13' aluminum boat worked well with an 18, not with a 9.9 tough.

 

The standard prop supplied by Evinrude for the 18hp was a 9X10,I chose the 9X9 too make sure it got out of the hole.Probably sacrificed top end speed and fuel consumption a bit,
I just need it to get me out fishing,and the 9X9 will probably let it plane at a slightly slower speed.

 

Yes, it may be enough to get it on plane. They were very good motors, simple, light, plus parts are easy to find and cheap.

 

Obviously there are huge differences in the local boat fleets, but in Finland most 14' from 70's and 80's had a 20 hp maximum, some up to 40 hp. No difference between aluminium and grp. Both had the same weight 140-250 kg. Now there are boats with 60 hp and even higher maximum, but they typically weigh 300 kg or more.

 

I have always imagined Finnish boats to be pretty tough and reliable,mainly because of the weather.(and the craftsmanship of course)
My old Dad was up there on Destroyers in ww 2,he said it was a pretty hard enviroment.

 

Just browse the forum and seen this...

Why trying to get the smallest motor on your boat to do the work as per OP.
Boats are much like air-conditioning. A larger unit than required always do the job better and more efficient. Likewise with boat engines. Get the largest you can get within your budget without going for an overkill.

I always had a bit of more hp on my cabin fishing boat than the minimum required and never regretted that. Gets easier on the plane with loaded boat, run on the plane at small throttle opening at an ecconomical rate rather than a smaller unit at a thirsty large throttle opening to do the same.
I had to run away and through hell of storms, winds etc during my fishing days and never regretted the extra muscle so to speak to get out of troubles.

Just my take and something to ponder about. Stywe lyne

 

True to some extent, but you can generally get better mpg by not going to maximum recommended hp, unless there is going to be constant heavy loads. Judicious selection can save around 25% in fuel cost, avoiding the big bangers. But you'll be cruising slower. I am amused to see old boats from the 70's and 80's, that would run quite well with 150hp outboards, being restored and then re-powered with much more, maybe even 2x150 hp, this is a bit dumb, in my estimation, if it means you are compelled to put up with a higher planing speed, lower mpg, and less leeway to back-off in bad conditions, for the sake of extra speed on the rare day you can use it.

 

Hi Wynand,thanks for your interest.
I can see your point,I have a few reasons for wanting the setup I have got,the main one is
I am 71yrs old and need a lightweight setup that I can trailer and launch.The other is storage space and the fact that I have to take the engine off after each trip for security reasons.I am 40 miles from the sea,so it is a long day getting there and back.Plus allowing for tide times and the weather.When I test this 18hp I may decide on a 30hp(with a key start)setting the throttle and using the pull start can be a bit dodgy at my age.
I noticed you are a steel boat builder,I was an apprentice boilermaker in th early 60's in Whyalla South Australia,happy days

 

Yamarin 4410 was tested with Yamaha F20, F30 and F40 in 2001 by a Finnish magazine "Vene". The boat weighs (empty without engine) 300 kg and LOA is 4.35 m. The test was done with two persons and test equipment on board, about 200 kg in total. Some picture of the boat.

The top speeds were 23.1, 27.3 and 31.1 knots. At 20 knots F20 was still at 5500 rpm and F30/F40 at 4500 rpm. The fuel consumption was almost the same, F40 slighty less than F20 and F30 slightly less than F40.

But the question was about traditional 2 stroke engines. Those have typically quite bad efficiency at low loads and I would say typically the bigger engine is less economical at the same speed.

About the same data and a lot more can be found from Yamaha's database "Körfakta". Unfortunately it is in Swedish, but you probably understand the figures. Here is one from 2003 including Yamarin 4410.

Note also Yamarin 4210 (4.18 m, 175 kg) tested with 9.9 and 20 hp 2 stroke engines. 9.9 hp was clearly more economical at 16 knots and it planed with two on board (but not 4).

Flipper 630 OC was tested with Yamaha 90 and 115 2 stroke. Yamaha 90 was clearly more economical at all speeds and loads tested.

Here is the 2016 Körfakta.

 

Nice pics and info J,thanks.
I must admit,after years of comparing motors,I do favour the Yamaha,with Evinrude coming a close second.
I was into motocross for a while,Yamaha and Honda were always the best.(two strokes)
I always thought Suzuki didn't quite make the grade,I think they cut corners on the quality
and paid the price for it,they seem to do it even with their vehicles,nice and shiny at the onset,but high on maintainance later.
I think I will go for a 30hp Yamaha 2 stroke,from your specs and the fact that I won't be carrying heavy loads they look the favorites.
When I got this boat the previous owner had removed the levers/controls off it,it had had a Yamaha 15hp on it.Luckily the steering was intact.
I converted it to an Evinrude setup.Looks like it might be back to the drawing board.
My aim with the boat is safety,reliability,and being able to tootle about on the water doing a bit of fishing.(old age creeping in Haha)