direct surface drive questions

[kfly]

I've been a long time viewer of these forums and learnt much and now I have a few questions about a drive idea for a boat I plan to build.

I'm about to start construction on a 24ft vessel with 7ft beam with variable deadrise (around 17 degrees down to 12 at the transom) displacing 1350kg. I'm really hoping to run a direct drive diesel (145hp 3600rpm) to something like a 12 x 17" surface piercing prop which in theory would give something around 41.5 knots at 15% slip.
Now my questions:
Is that enough power to turn that prop?
Does that sound like the right size prop and is there anywhere that makes such props?
Will the engine start basicly locked in gear like that?

And before I start to look at things like shaft bearings, seals, universal etc. Am I basicly kidding myself?

[Tim B]

First of all, get your requirements written down. E.g.

Where do you intend to sail? Sheltered water, inshore or offshore?
How many people do you expect to carry?
How much luggage and cargo?
Intended design speed?
Dimensional constraints?
Weight limit?
Budget?
Styling?

There are many more questions of course, but these should get you started. Once you have done that, get a first-shot hull drawn. This will allow you to do a hydrostatics prediction and a resistance prediction. With the resistance prediction, then you can look for a suitable propeller and engine to power the boat with, and talk to engine and propeller suppliers to find out the properties of the available parts, you should then be able to match the engine, propeller and hull, so that you have some degree of confidence that you are in the right area first-time out.

Formalising what you are doing is not a bad thing to do.

Hope this helps,

Tim B.

[Frosty]

Seafury,--one of the worlds surface drives companies are in New Zealand.

I assume NZ means New Zealand.

You will never ever attain only 15% with an Surface D

Google Seafury they have a very helpful web site.

If your realy serious about this tell the prop manufacturers what you want to do . They will tell you what Prop to buy , what gearbox and what engine will drive it.

If you fit and engine and gearbox (like everyone does ) and then ask the prop man to get you out of trouble the prop man wont like you very much.

[kfly]

Originally Posted by Frosty

Seafury,--one of the worlds surface drives companies are in New Zealand.

I assume NZ means New Zealand.

You will never ever attain only 15% with an Surface D

Google Seafury they have a very helpful web site.

If your realy serious about this tell the prop manufacturers what you want to do . They will tell you what Prop to buy , what gearbox and what engine will drive it.

If you fit and engine and gearbox (like everyone does ) and then ask the prop man to get you out of trouble the prop man wont like you very much.

Thanks for your response.

Yes NZ is for New Zealand.

I thought 15% might of been a reasonable prop slippage for a highish speed set up. Do you think It would be more like 20-25%?

I've had a look at all the major surface drive manufacturers including Sea Fury, I believe they use the Diamond Back props the same as Levi Drive. The smallest being 17" in diameter which I suspect would be too large. This would seem to be the problem with most of the surface prop manufacturers I can find. I have been thinking maybe someone like Ron Hill could make up one of his cleavers to suit?

I realise almost everyone uses a gearbox but I have seen it done without plus I don't like doing things just because most everyone else does. I launch straight out in to open water and really have no need for a reverse gear. I thought I may be able to save the cost, weight, and hassel of a gearbox by coupling a low reving diesel straight to the prop shaft via a universal joint or something.

[Frosty]

Surface drive propeller sizes do not follow conventional sizing. You fit the biggest propeller you can swing.

Remember only half the prop is in the water so you need twice the size at least.

Im using the diamond backs and they are now old in their design.

Living in NZ you will be able to try many different sizes and shapes on trial basis, (you damage it you buy it)

Yes you could direct drive --why not.

[tom kane]

Build a trimmable direct drive it`s simple and easy and solves launching problems plus many others.

[Frosty]

A trimmable drive offers enormous problems compared to fixed, thrust, steering rams etc and has no or little trimmable effect on the hull itself but just to unload the prop under take off that a correct sized propeller vented with exhaust wont have.

If the boat is to be left in the water for any length of time fouling rams and pipes will be constantly in need of attention.

Home made trimmable drives rarely look professional yet the simplicity of fixed can.

[tom kane]

Your concept of a trimmable drive Frosty is well of the mark. If you want to create those problems then you will have them. A trimmable drive can be as simple as a fixed drive and even more beautiful if you want it to be.Do you remember the old Zip drive that were once available on some classic boats?

Long Tail Boats are very popular and defy some engineering principles.
Trimmable drives can offer more of the practical needs for a boat.

[Frosty]

I don't think he is wanting some Heath Robinson Thai type long tail with a engine out in the air.

Im not talking hypothetically about something that may work, but something practical that can be fitted to a nice boat and have some value.

A trimmable surface drive has very little affect on the of trim the boat.

The surface drive will be 8 degrees or less, raising the smallest amount above the propellers optimum point will cause slip and waste of power.

To alter a boat trim involves propeller rake.

There is nothing more simple than a fixed drive with the right prop in the right place and a simple rudder with inboard ram as is the Sony surface drive with vented exhaust

If you don't vent the props you will need to raise it and cause aeration to get the engine to HP

Right prop in the right place makes trimmable not necessary.

[whitepointer23]

sorry to butt in, but i have always wondered about surface drives. what are the advantages and dis advatages. i find it hard to understand why you run a prop 1/2 out of the water and why it doesn't just sit there cavitating. dumb questions but if i don't ask i don't know.

[Frosty]

All you ever need to know.

http://www.titanic-nautical.com/Ship...ropellers.html

[whitepointer23]

Originally Posted by Frosty

All you ever need to know.

http://www.titanic-nautical.com/Ship...ropellers.html

thanks, that is a good article.

[Tim B]

I think the author is confused between cavitation and ventilation. They are not the same thing. Other than that, a decent pro-surface drive article. SDs are not the answer in all cases, but here are the major pros and cons

Basically, the PROs are:
Better efficiency than other options at very high forward speed.
Better trim control (though this is debatable).
Easy engine mounting.

The CONs are:
Traditionally, hideous efficiency at low speeds.
Slight misalignment will cause big vibration issues.
Massively noisy and easily detectable (if this is an issue).

So, for low speeds, a large submerged propeller; for medium speeds consider a waterjet; and at very high speeds, use surface-piercing. Your definition of low, medium and high will depend on what you're designing.

Tim B.

[tom kane]

Originally Posted by Frosty

I don't think he is wanting some Heath Robinson Thai type long tail with a engine out in the air.

Im not talking hypothetically about something that may work, but something practical that can be fitted to a nice boat and have some value.

A trimmable surface drive has very little affect on the of trim the boat.

The surface drive will be 8 degrees or less, raising the smallest amount above the propellers optimum point will cause slip and waste of power.

To alter a boat trim involves propeller rake.

There is nothing more simple than a fixed drive with the right prop in the right place and a simple rudder with inboard ram as is the Sony surface drive with vented exhaust

If you don't vent the props you will need to raise it and cause aeration to get the engine to HP

Right prop in the right place makes trimmable not necessary.

I think that you have given many of reasons why it is necessary to have a trimmable drive and why most people who have used fixed drives are never happy with them.

[Jack Hickson]

Mounties tried surface drive on their catamarans here. The balance had to be absolutely perfect. The only way to get it good enough was to ship them to France. Cost $17,000 a prop.