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#11
posted to rec.boats.cruising
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Sailboat propeller question
Dennis wrote: Hi Roger and everyone who replied! Thanks so much for the constructive feedback! Couple of points to clarify: - I forgot to mention, the gear ratio is about 1:2.06 (can't read the I think your gear ratio should be closer to at least 3.0. Your transmission sounds like it is configured for a planing power boat. Sherwin D. label on the gearbox) - I recently had the bottom done completely - it is as smooth as it gets. - I also had the propeller checked by the manufacturer and they confirmed that the pitch is what it has been manufactured at. - I have a brand new shaft and coupler and these are properly connected. So, I guess my best bet is to check the transmission. Now, how do I do that? - Can I check the clutch/transmission without disassembling everything? Any ideas? - Can I check and repair the transmission without hauling the boat out of the water? - I also notice that there is a very high pitch noise from the engine after about 3700rpm until the max. rpm. Could that also indicate a toasted transmission? Thanks again! Dennis |
#12
posted to rec.boats.cruising
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Sailboat propeller question
"sherwindu" wrote
I think your gear ratio should be closer to at least 3.0. Your transmission sounds like it is configured for a planing power boat. No, that's a fairly typical reduction for a sailboat. The greater the reduction, the larger the prop. Sailboats usually compromise effeciency under power with smaller reduction ratios and props in order to reduce drag under sail. -- Roger Long |
#13
posted to rec.boats.cruising
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Sailboat propeller question
28 foot boat so waterline length about 23 feet.
7800 pounds displacement, probably from brochure, say 8200 really or 2.31 long tons. 10.5 horsepower, probably 9.5 after charging batteries and with transmission and shaft losses. With the right prop, you should easily be getting 6.5 knots. 3500 rpm (I can't imagine you want to run that fast) / 2.06 = 1699 shaft RPM Va say .87 x 6.5 knots = 5.66 knots so a skinny two blade prop should about 10 inches in diameter. You are right on the money. A 10 inch prop with these numbers should have a pitch to diameter ratio of 1.15 pr 11.5 inch pitch so you are a bit light on pitch. Efficiency would be about .36 with the right wheel. Your numbers (including the lightly pitched wheel) are all pretty typical for a sailboat. There is something else going on here. -- Roger Long "Dennis" wrote in message oups.com... Hi! I've got a 28 foot sailboat, 7800lbs with a 10.5hp engine. It has a 3 blade prop with 10 inch diameter and 9.3 pitch. Prop is in like new condition, so is the bottom. When I sail, I can push her up to 6 knots. Though, when I spin the engine at 3600 rpm I only get about 3.5 knots out of the boat. The engine easily manages more than 4000 rpm, so I don't think it's an engine problem. I suspect that the prop has the wrong pitch, but according to the manufacturer it's exactely the right one. Why don't I get more than 3.5 knots under power? Does anyone have a similar configuration or any clues? Thanks! Dennis |
#14
posted to rec.boats.cruising
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Sailboat propeller question
The analogy with an outboard may not carry over to an inboard, but on my
sailboat there is a distinctive improvement in performance using a 'pusher' type engine gearing. Most outboards that do specify their use on sailboats usually have gear ratios 3.0 or higher. The best pusher I ever used was my British Seagull 6 bladed prop with a gear ratio of about 4:1, and I saw pictures of this engine pushing huge heavy barges. Roger Long wrote: "sherwindu" wrote I think your gear ratio should be closer to at least 3.0. Your transmission sounds like it is configured for a planing power boat. No, that's a fairly typical reduction for a sailboat. The greater the reduction, the larger the prop. Sailboats usually compromise effeciency under power with smaller reduction ratios and props in order to reduce drag under sail. -- Roger Long |
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