As the post title says... What is the best prop for extra thrust for a 102 longshaft motor pushing a 21ft yacht? I have a four blade rather large prop at the moment around the 12" mark... Would the slightly smaller five blade weedless prop from a century fit and would it give better thrust than the four blade prop that is already on the old girl ?
Is this a 102 plus? If so you have a larger slower running prop designed for pushing displacement hulls aka the "barge pusher" These would pull HMS Belfast out of the mud
I'd suggest that have an ideal engine and prop for your boat.
Whatever prop you fit a 102 is not quite man enough for the job, unless you plan on staying in a very sheltered area with no tide.
It will get you around the marina, but if your looking to get home when the wind & tide have decided that's not where they want you to go, you had better get some navigation lights.
The only way to find out is to try it out, you have not given us half enough info to even hazard a guess, how much windage, how heavy, how much drag, what sort of winds, how fast does the tide run & how good are your crew?
Make it idiot proof and someone will make a better idiot.
Had a 21 ft. sailboat, 2250 lbs. displacement, fin keel,spade rudder, shallow v bottom. Used a '71 WSPCL 5 blade 11" prop., tried a '56 AC 102 2 blade prop., and a Century bitsa, amal carb, 3.5:1 direct drive gearbox, long shaft, 9" 5 blade prop.
Similar performance. Out 3 kn.mi against ~2 knts current, back the same 45 min. each way. The best overall in my view mainly because of low weight for carrying was the bitsa. It was up a vertical wharf ladder at low tide, the '71 Silver Century was quite a lift. The '71 had the most push in a hard chance.
Because according to my calculations your doing 5.6 Knots in still water with a 21 ft heavy boat, that is not hanging about at all, that's about what I'd expect from a Kingfisher or a 170.
Make it idiot proof and someone will make a better idiot.
Hi Charles,
Not certain on the 2 kn. current figure but should be close, time about right, sometimes 50 mins. instead of 45. IIRC on waterline about 19 ft. 3 kn. mi should be within 10%, I should check chart. Oh, BTW, it was the good high comp. Silver century. Anyway, I'd be pretty confident of 5 kn/hr. That includes slowing motor, pulling the prop clear of the water,all too frequently to spin off some of the eelgrass that seems to be highly attracted to 5 blade Seagull props. Also easily driven hull, 3/8" mahogany taped seam hull, would consider it light displacement.
By the way, recently picked up what appeared to be a '49 Merc KF7, but turned out to have a 1950 KG7 powerhead. Also the other desireable goodies like the tiller handle screwdriver, and the quickchange prop shaft adapter.
