chosing the RIGHT seagull

timberman2004 · Post by **timberman2004** » Sun Apr 06, 2008 9:11 am

gentlemen ....and other seagull perverts

the boat in question is a 14 ft Kittiwake http://www.kittiwakeboats.co.uk
LWL 14' (4.3m)
LOD 14' 3"
Weight 705lbs
Beam 5' (1.5m)

the great unknown at present is the dimension top of transom to waterline ...and how critical this depth issue really is

I'm waiting for delivery with drooling anticipation ....as well as a selection of running, pumping, noisy, smoky seagulls to 'Try for Fit '
getting them all up and together seems to have led to this present addiction

I have the choice of a
1937/8 OP Marston,
1958 SJP,
1978 FPC,
1979 WSC,
and a lumping 1967 WPCL

I know they're displacement engines, but obviously looking for the one to push the boat along briskly and economically

I guess 1/2 the fun will be trying them ALL out

great to hear from anyone with experience in this area ...as well as anyone who has any Marston spares

Post by **charlesp** » Sun Apr 06, 2008 9:19 am

SJP sounds just about ideal to me...

Vic · Post by **Vic** » Sun Apr 06, 2008 10:08 am

I think I'd go for a clutch model rather than a fixed drive.
The recommendation in the link is for a long-shaft engine of 3-4hp therefore it probably means it will have to be the WPCL.

The power you require will rather depend upon where you will be operating and in what conditions. In tidal water or a fast flowing river you need enough power to be able to get the boat up to its max hull speed. Against a stiff breeze you need the power as well.

The transom height is critical in as much that if the prop is not sufficiently deeply immersed you loose drive. The alleged difficulty in starting if the prop is too deeply immersed is IMHO less critical.

rosbullterier · Post by **rosbullterier** » Sun Apr 06, 2008 1:34 pm

I thought the way to go now is with 'surface propulsion drive'.
Apparently there is no cavitation when the hub is at water level with just the bottom half of the propellor driving.
Plus there is a 20% power gain and 15% economy gain . . .

timberman2004 · Post by **timberman2004** » Sun Apr 06, 2008 8:01 pm

Hmmm ....just trying to work out the physics of that ...

only utilising 50 % of the props capacity ???

cavitation should surely be worse as each prop blade strike is introducing more air ???

mrdraddy · Post by **mrdraddy** » Sun Apr 06, 2008 8:15 pm

Forget surface drive etc(totaly different kettle of fish)think charles is right

rosbullterier · Post by **rosbullterier** » Sun Apr 06, 2008 9:35 pm

Well just google 'Surface Drive' or 'Boat Design Forums' and read the reasoning.

They say Surface Drive has been around for ages. Its not just used for power speed boats but on heavy commercial displacement vessels.

How about trying with an 11" seagull propellor. Or better still, one of those huge 102 barge push props with four blades.

The big surface drive propellors work better at lower revs than performance submerged props.

So about 4500 RPM would be ideal . . .

CatiGull · Post by **CatiGull** » Tue Apr 08, 2008 11:39 am

My forty plus pushes my 500 pound Island 17 to hull speed quite nicely as a data point for you. My Forty Featherweight can do this in good conditions but not if we are 4 up or are fighting a Force 10....