auxilary engine, what size??

[matt h]

got a sr5.4 what would be the horsepower in need to look at to keep the boat moving in the event of main engine failure,those Yamaha 2.5 look ok at 12 kgs!! would this engine keep me moving forward??

[Polwart]

Originally Posted by matt h

got a sr5.4 what would be the horsepower in need to look at to keep the boat moving in the event of main engine failure,those Yamaha 2.5 look ok at 12 kgs!! would this engine keep me moving forward??

Do a search - its a well discussed topic here. But the general rule of thumb is 1HP per meter of boat.

A wee 2.5 HP engine might move you about ok on a nice calm day - but so would a pair of oars. The general message from previous responses to this question has been 'when you need an aux you need one with some serious power'.

[Nos4r2]

4hp minimum-if you have the flooding hull sealed. 5hp min otherwise

Personally I'd go for 5 or 6 if there's room.

[BogMonster]

An interesting question with an aux seems to be whether you can get all the power out of it...

I have a 6hp Johnson which is the same engine as the 5 and 4 in the range just a different carb etc to give more power at the top end, but because it won't rev anywhere near the maximum, I'm now not sure I am actually getting any more than I would have done with the same 4hp. Going to try and get a finer prop for it to find some more revs - I reckon I am only getting about 3500-4000 revs on the aux at full throttle. I hope that with a finer prop (if one is available) I'll be able to get somewhere up around 5500 which will find the missing horses!

I filled it up with minced kelp this afternoon and I can confirm it doesn't produce anywhere near 6hp when trying to digest a small kelp forest

[Lawrence]

I've got a Ribcraft 5.3 and I replaced my Evinrude 3 hp short-shaft auxiliary (2 stroke) as it struggled to push us along, only reaching about 3 knots into a breeze in fairly calm water. I was going to get a Johnson 6 (4 stroke) but the list price was about twice as much as a Johnson 4 which is essentially the same engine.
So I bought the 4 long-shaft and, apparently, the only difference between the 4 and 5 hp models is a throttle restrictor plate which if you turn it over says 4 on one side and 5 on the other; the 5 side letting the engine rev more. So I had it turned round and early tests suggest I should get 5 knots when it's run in.

[BogMonster]

Damn, wish I had known that!!!

[jwalker]

Originally Posted by BogMonster

....I hope that with a finer prop (if one is available) I'll be able to get somewhere up around 5500 which will find the missing horses!...

Get the file out.

[BogMonster]

Originally Posted by jwalker

Get the file out.

File my boat down till it is a bit smaller so it will go quicker?

[jwalker]