What Outboard

[Baz]

Hi all, Firstly let me say, I know nothing about outboards at all, so am asking for your advice as to what my son will need for his boat. The boat is a smallish day boat of 16ft long, and 6ft 6in wide, a small cabin area with a smallish rear cockpit. The boat is steered from a conventional wheel and cables, as will the throttle and gearchange. Now, will he need a longshaft outboard,shortshaft,2 stroke, 4 stroke, and of what power. He hopes to take it out for a bit of fishing, and of course to give the family a trip up the river occasionally, on the River Great Ouse.

Any suggestions or advice would naturally be accepted with gratitude.

Baz

[w44nty]

What make of boat is it ,that will decide what engine it will require in shaft length,I would suggest a 10-15 hp ,pref 4 stroke as it will be more quiet and cheaper to run,it is very important that the shaft length is correct.

[Baz]

Hiya w44nty, The boat was built by Yeoman Cruisers in 1975, but what the depth of the boat at the back is, I don't know,I'd have to phone my son for him to measure it, but thanks a lot for your reply, most appreciated. The actual power of the engine, I thought of, was, as you suggest, about 10 to 20hp, great minds eh.

Baz

[senator]

when you say take it out fishing are we talking river or sea?

10hp may be fine on the rivers but you may want a bit more at sea.

As to the length of shaft most boats of that length will be long shaft, short shafts tend to be more used on dinghys and ribs which have a lower transom but that said there is only 3" between them.

[Baz]

Hello Senator, nice of you to reply. I'm sorry, I should have said fresh water only, The River Great Ouse to be precise, and it's tributaries. The diagram supplied by Strowager is spot on for what I was looking for, and is now in the hands of my son. I'm of the opinion, as is my son, that the power should be in the area of about 15hp.

Thanks once again.

Baz

[Guest]

Hiya dad

well i measured the transom and it is 17" some what it shows on the diagram is a short shaft correct ?

[Baz]

Hiya Ian, welcome to the site. At 17", I would say it's probably on the limit of a short shaft, and as there's only a few inches in it, I'd go for the long shaft. If I'm wrong with my judgement, then I'm sure someone will be along to correct me, as I've already stated that I'm not up and running with outboards.

Enjoy the site Ian, there will always be someone about to help you, and nice people at that. I reckon that plug deserves a pint eh.

Baz

[w44nty]

I would strongly recommend you go for a long shaft,it will make your boat easier to handle and will be a dream,with a short shaft your boat would probably wander a lot and also cavitate,the long shaft will make her better all round,even mooring will be easier.

[Guest]

If it's 17" from the top of the transom to the bottom edge of the boat, that's awkward, as it's in the middle between long and short shaft, (15" and 20").

As it's for river use only, and therefore displacement speeds only, I wouldn't go larger than 10hp. Any more power will not get used, and will only make manhandling more difficult.

I concur with Senator's remark, a boat of that size is more likely to have been built for a long shaft.

If you fit a short shaft, the 15" length will mean that 2" is "blinded" by the 17" transom, making it likely to cavitate and giving it less grip in the water.

The extra 3" sticking down below the bottom will not introduce too much extra drag at displacement speed, but if it had been a planing boat, then it would have been too long.

(oops, took too long typing this, I see that Baz and W44nty have now covered these points.)

[Antares_9]

One point worth considering, especially on a two cycle is back pressure, if the hub exhaust and relief exhaust are too far under water then it will affect the running of the motor at low speeds as a two cycle has a lot more exhaust gas per revolution to get rid of and it's not forced out in the same way as a four cycle, relying on open port timing and gas speed rather than an actual exhaust stroke. This is why you will see the relief exhaust above the waterline operating with very little coming out of the deeper hub exhaust when stationary on tick over.

[Guest]

hiya

thanks for the welcome, i think i will go for a long shaft

more choice, better start saving now, oh and dad

i owe you more than a pint !

[Baz]

Ian, you old smoothy you, ahhh shucks.

Baz