My old Evinrude 15 hp.. Again

[Patrik]

So. I was out playing with my Aerotec 420 and my old 1978 Evinrude 15 hp. Everything felt good, even though I feel it's a bit slow at 14 knots on full throttle.

However, all of a sudden the fun ended, as the engine stopped working at high rev. The engine sounds just as usual, but on high rev it seems to stop being connected to the prop. Works as usual on slow speed. I turned right back and turned it off, however I had to go for a while to get back to my cabin.

So what can this be? Is there some gearbox or something in these that has broken down? I've got a feeling that this will be too expensive to fix...

I'll be happy to dissect the engine if I have at least a little clue what to look for.

[Patrik]

Now I've gotten tips that it might be the prop hub that's gotten spun or something. Looking into props. Ideas are well welcome!

[Patrik]

Now I've been looking up spare parts. I thought I might wanna change the prop too while I'm at it. Am I right that the boat can go faster if I go from a 9,5" X 10" to a 9" x 11"?

If the difference is not notable, I wont waste my money.

If I should go for it, should I choose 3 or 4 blades?

[9D280]

You have answered what I was going to say - I had an old Johnson 25 - 1972 vintage which we left one morning at a good speed, and by the time we got halfway back it couldn't even keep the boat on the plane! If you look round the hub, you will probably see small traces of extruded rubber where the slip has melted & expelled it.....

As for pitch & diameter there is no easy answer - you may need to go down an inch in pitch. Problem with tiller control (I assume?) is that you are likely not going to have a tacho, so for all you know you may have been nowhere near max RPM. Can you borrow a prop? One thing is for sure - if you go down in diameter the drag reduction can be compensated for with higher pitch, but at risk of it "letting go" more easily. An experiment is usually the only way...

All 4 blades will do is give you more grip, (but at the expense of a bit more drag) so unless you are regularly towing big heavy boats or needing to get six skiers out the water on start, I'd stick with three blades.

[Patrik]

Oh yes you are absolutely right, I took the prop of yesterday and the rubber in it was crumbled and almost fell out just by looking at it. I don't bother doing anymore research about other possible errors...

I saw this video on youtube with someone swapped to a 11" pitch, looks good to me?

‪1983 Johnson 15 on an 11' inflatable‬‏ - YouTube

However, the 11" wasn't in stock on boats.net, so I might go for something else. Since it takes so long to ship to Sweden I wouldn't want to wait for it to get back in stock.

And now I hear that you say I should go DOWN in pitch instead of up? :o You are right that I have the tiller and no idea of RPM. I don't know if I should bother with getting a RPM meter, I actually rather just try and do an educated guess. However, I'm not educated and feel like I'd rather get some tips. So you think that lack of RPM is more likely to be my problem with speed, rather than lack of pitch?

[9D280]

Possibly. Boat engines are a balancing act - the prop is the equivalent to the final drive (differential) on your car. By making the pitch bigger, in theory you are geared higher, so on paper will go faster......IF you have enough horses / torque to overcome the extra force that is required to push it all along (think of gears on a bycicle - you can go faster in high gear, but will slow down quicker when you run out of breath to pedal on a not so steep hill.

So, this is where the rev. counter comes in. If your engine is rated to, say, 5500 rpm max, and flat out you are only getting, say , 4500rpm, then your propellor is too high a pitch. By pitching down, it's like going down a gear, and so the engine can turn it more easily, and so rev. higher. The loss in pitch is usually more than offset by the higher speed it's turning at. You can et cheap battery poweered tachos

"non specific" ways of looking at this - was the engine labouring or screaming? Are there other sibbers out there with boats you can compare? (plenty of posts on prop pitch on here). Somewhere out on t'net is a prop guestimator (use it with a pinch of salt, as the hulls are all American, so you have to guess what might be similar to your boat).

Do you have a dealer or a repair shop nearby who could maybe lend you a prop or two to experiment with?

[Patrik]

How do I go about to measure with the tacho when the prop is in the water? Is it possible to measure of the flywheel or something? I thought I saw that said somewhere, but is that thing really going as fast as the propeller? Sounds weird to me

I understand what you're describing here. I just saw that guy on youtube pitching up the same engine and a similar boat, except my SIB is 14 feet instead of 11. And he seems to go a lot faster than I am with mine. But I will take a better look at threads on this forum.

The lending seems to be difficult, I actually rather have to buy two from the US or something, as the props are crazy expensive here. £150 compared to like $50 in the US. Don't know how it is in the UK? I'd like to get it for under 500 swedish crowns (so the $50 + $18 shipping is fine) to get it through customs without having to pay toll.

As a true amateur, I would say that the engine goes as fast as possible on almost full throttle (10-20% from it). Giving it full throttle doesn't add much speed. This is why I thought it should have a bigger pitch.