distance between the hull and the propeller.

hi,
i bought a 9.9 volvo penta SD(sail drive) engine for my 8 meter sailing boat. and i will install it.
there is not a standard guide for installlation. Depending on the heigth of the engine bed, the propeller can be very near to the hull bottom, or can be fairly apart from the hull.
which case will be more effective?
cheers

 

Normally the skeg plate should be level with the hull bottom at the rear, so when the motor is down the prop would 'look' under the hull. The distance the prop is from the hull only has an effect on planing hull's handling. In your case you won't notice. The correct height should be where the skeg plate just planes on the water when under way, but should not be submerged or out of the water either.

 

this is the boat.
and the sail drive type engine, is about 1 metre ahead of the skeg.

this the picture of the volvo saildrive engine.
depending on the engine bed, the distance of the propeller from the hull bottom changes.
what should be the distance of the propeller from the hull bottom?
if i know that i can design the engine bed according to this.

best regards

 

Dave Gerr gives the following values in the propeller hand book.
Minimum clearance of the propell diameter:
RPM 200-500 S/L less than 1.2 = 8%
RPM 300-1800 S/L 1.2-2.5 = 10%
RPM more than 1000 S/L over 2.4 = 15%
Planing crafts S/L over 3 = 20%

 

thanks teddy,
but,
is S/L, sail area/length ratio?
and what does %8,10,15 means?
is it clearance of the propeller from the hull bottom/propeller diameter?
cheers

 

Urisvan, your prop should be as deep as you dare place it. This is considering you don't have a fixed keel to offer protection in the shoals. On a hull shaped like that, you'll probably have to drop it in under the bridge deck (if it has one) or under the forward end of the cockpit. This will place the leg fairly deep and dramatically increase your minimum draft requirements.

Divide the diameter of the prop in half and use this as your minimum distance from the bottom of the hull. This distance is measured from the blade tip to the centerline of the hull, directly above it. It will not be quite as efficient as if it were deeper, but it will provide you with the least amount of draft you can live with, without affecting performance significantly.

A short, low resistance skeg would be a good idea to protect your investment.

 

thank you Par,
because not being instaled yet, it is not seen in the picture, but i have a fixed keel. So do you mean that,
i should place the propeller as deep as possible?
cheers

 

If you have a deep fin going on, place the drive leg the full depth possible. The prop should be well clear of the bottom of the fin, in the event of a grounding. The prop should swing at least 6" above the lowest point of the fin, to prevent it eating a lot of bottom, when trying to back off after a grounding.

It's entirely likely you'll have plenty of room below the leg to fin bottom, if it's of reasonable proportions.

The only reason I can see you wouldn't want to use the full depth of the drive leg would be a shoal fin. As I mentioned, if you do have a shoal (shallow) fin, then insure the prop will swing well clear of piled up mud and bottom debris in the event of a grounding.

 

thanks

thanks par
it is all clear