Saildrive..

[gps999]

Can any of the learned forum contributers explain to me in very basic terms what the ratio of a saildrive means.. ie

I have an engine that has max rev of 2600rpm and a saildrive with a ratio of 1:66:1 this obviously spins the prop at a certain rpm ie X

If I change the engine to one which has max revs of 3800rpm with the same saildrive X will be much a much higher number.......

Some providers offer sail drives of ratio 2:2:1 or 2:15:1 I can't really get a straight answer from anyone to explain anything

My simple brain says up the revs to 3800rpm there needs to be a reduction in gearing which results in X being the same as it was with the old engine and saildrive..

One suggested solution which leaves me puzzled is keep the existing saildrive at 1:66:1 and limit the max revs of the new engine to 2600 rpm !!! I know this would work but what about the extra power I am missing and isn't revving an engine a useful way of ensuring it doesn't coke up etc etc

Please keep it simple...

 

[VicS]

Yes the ratio is the ratio of the engine rpm to the prop rpm
So with 1.66:1 reduction ratio at 2600 rpm on the engine the prop will be turning at 2600 / 1.66 = 1566 rpm

If you fit an engine with max speed of 3800 it means that prop speed will be up to 3800/1.66 = 2289rpm.

To keep the prop max rpm at 1566 rpm you would need a reduction gear of 3800/1566 :1 ie 2.43:1

However changing the gearbox ratio is not the only way to achieve the same thing. A smaller pitch prop could be used.

BUT if you are fitting a more powerful engine you will probably be needing a larger diameter prop as well if it is possible to fit one.

 

[gps999]

As ever there is always a supplementary question. Thanks for the above that makes very clear sense..

So if I keep the existing drive at 1:66:1 and adjust the prop size and pitch theoretically it would be fine. However can anyone say whether running a sail drive (which was rotating the prop at 1566rpm) at the higher speed of 2289 rpm would be advisable ?? its a 110s is there a max recommended prop rpm for this drive?

If the replacement engine is similar in power output, is it better to just aim for the nearest saildrive gear ratio to 2:43:1 and keep the existing 2 blade folding prop?

 

[Tranona]

The 110S as you know is out of the ark, and if you are changing your engine you will probably be better off buying a matched engine and saildrive. The latest Volvos rev to 3200 and have 2.1:1 reduction (130 leg). You will then be able to use a standard Volvo prop (or an equivalent folder). The older 20 series engines rev to 3600 and have 2.4 reductions (120 legs). Both give shaft speeds of roughly 1500rpm so can use the same size props.

Most other small hp (20-30hp) engines rev to 3600 so the saildrives they offer (usually ZF or Selva) will have roughly 2.2 - 2.4 (have not checked exactly). From memory, Yanmar are also 3600/2.4 - but whatever it is, again you will have a matched prop size.

Not sure that you will find a modern engine to mate with your 110, nor a prop small enough to run at 2200 using your reduction.


Hope this helps.

 

[gps999]

Thank you for your replies. This totally bears out my thinking but with the added benefit of now understanding the mathematics.

I ve been looking at re engining and have approached a number of engine suppliers who have been very happy to supply. But when I raise the issue of gearing they become slightly evasive, one even suggesting the limiting the engine to max 2600rpm as an option.

I will therefore be "biting the bullet" removing the fittings from the ark and getting engine and saildrive.

Having revisted my figures I have the following:- engine max revs 3600 gear ratio 2:15:1 max prop speed = 1674rpm which is only 100rpm more than existing set up and I would rarely gun the engine at max revs..

So I can use my existing prop...that is if the fitting is the same..

Thanks again

 

[Nostrodamus]

One suggestion is to run what you intend to put in past some prop manufactures and they will tell you if it is possible or like me they suck in a lot of air and talk about cavitation.
I have a similar problem in that I have a volvo MD22 which runs well and has very low engine hours. The ail drive leg had to be replaced because of corrosion and they no longer make the 120 leg. A 130 was fitted but these have a higher gear ratio as volvo expect them to be matched to their lower reving engine.
It took an age to find some kind of prop that would work. I now have a prop that goes fine in reverse but it takes an age to dig in in forward.
What difference would increasing the pitch in forward make. I am told I will get better drive but loose revs from the top end and may get cavitationif I really need to push the revs up.
It is all a black art to me.

 

[Tranona]

Be wary of buying a saildrive from other than Volvo or Yanmar as you may find you are limited in your choice of props and rope cutters (if you want one) are only available for those two.