shaft length, engine size and prices

[BogMonster]

I know this is a dumb question but a search hasn't immediately revealed the answer to it even so...

Looking at what to get in the way of an aux engine for the RIB.

How do I decide/measure/work out whether I need a standard or long shaft engine? How long is standard and how long is long? Seems to be one of those things that I presume "everybody knows the meaning of" but isn't defined anywhere, or not that I can see anyway! I sort of assume that standard is for where the engine is the main engine on a small inflatable and long is probably for where the engine is fitted as an aux on a bigger boat, or am I barking up completely the wrong tree here?

Looking at Mailspeed Marine (not necessarily where I am going to buy it from, just that they have a selection of different engines on there with prices shown) there is a Honda 4 stroke 5hp for £800 and a Mariner 2 stroke 5hp for £729 so it seems to me to be worth a few quid extra for a 4 stroke - are these prices reasonable? they are about what I was expecting. I think the Honda looks like quite a good bet, bound to be fairly reliable if it is like everything Honda make on land, and I guess a 4 stroke is likely to be more reliable than a 2, which is probably a good thing in an emergency engine and would mean no messing with mixing fuel up

Finally are there any makes of engine that are particularly "narrow" in physical size - it is pretty tight on the transom between the main engine and the tube and really the smaller the physical size of the aux engine the better. That Honda looks pretty good in the photo.

Thanks

Stephen

[roycruse]

Shaft length is measured as the vertical distance between the top of the mounting bracket (ie the bit that sits on the top of the transom) and the Ventilation (cavitation) plate.

Seeing as the Ventilation plate usually sits around about level with the keel then it is also the vertical height of the transom



A short shaft engine being 15", a long shaft being 20" and an extra long shaft being 25"


With an auxiliary engine though because it is being mounted off to one side you can usually get away with a shorter shaft as the transom will be shorter there due to the V shape of the hull. so as long as the engine is long enough so the ventilation plate clears the bottom of the hull - and the prop and water inlets are in the water when the boat is in displacement mode it should be fine.

[BogMonster]

Many thanks, an excellent diagram too

I'll get my tape measure out but I'm pretty sure a short shaft will be OK based on that, the transom is not very high where the aux would be mounted.

Would you get any benefit in terms of thrust, from using a longer shaft one which had the prop even further down into "clear water"? -- just wondering if a long shaft might be more effective at the same HP, given that they seem to be about the same price.

[roycruse]

I don't actually use an aux so this is just my semi educated guess.

As long as the ventilation plate is as low as the hull at the point where your engine is mounted then your prop will be in clear water flow.

The only time a longer shaft might be useful is if you are in conditions that due to the boat being tossed about the prop is being lifted out of the water causing ventilation regularly.

Only thing you can do there is go have a look over the back of your boat when stationary in some waves - if you keep seeing the base of the transom clear of the water at the point where the engine is going to be mounted then a longer shaft might be useful.

P.S. I didnt draw the picture - I nicked it from another website