Pulling the trigger on new motor...help please.

[NCboatrx]

Originally Posted by a64pilot

My boat will make 8 kts or slightly more in flat water.
Hull speed is like 7.3 kts.
However I’d like a bigger motor. My reason is because at 1500 my motor sound wise is a delight, turn it up to 1900 and it’s not all that bad, but much over that and it sounds like mating chainsaws to me. I cruise at 1900 more than anything else to keep the noise acceptable. So I’d like a bigger motor, so that most of my motoring would be done at lower RPM and the noise, vibration and harshness would be so much less.

This is always a subjective issue as you may hear your engine's noise and vibration differently to the next person. There may be an issue with mounts, propeller or the actual boat structure and it may be possible to tune the installation with different flex mounts and a flex coupling or a different intake filter to help hide the noise. We have had issues where an engine has a "sweet spot" in a particular vessel and it's low (or high) RPM cousin just does not work as well. I wish that there was one hard and fast rule as this would really make my job very easy but I keep on learning as time goes on.

Stanley

[NCboatrx]

Originally Posted by Captain Randy

Stanley,
I have a Cheoy Lee Pedrick 41 weighs about 14T loaded. I am getting ready to replace the tired Perkins 4108 and considering a new Beta (and not a Yanmar) What recommendations and advice can you help me with in the decision. Is there a typical engine and tranny which is a relatively easy drop in and compatible with our Walters V Drive. Thanks, CR

Private message has been sent. Stanley

[Scout 30]

This article was posted on another thread & I thought it would be appropriate here. In my opinion most sailboats are under powered. There was a recent thread about exceeding hull speed & I think most well designed sailboats can do it. So, if you can exceed hull speed under sail why not under power? The truth is that sailboats are motored a lot of the time & most of that time it's straight into the wind. For me, having enough power to motor at 8 knots, instead of 4, when fighting a 20 mph headwind makes sense. And when it's calm just cruise at lower rpms.

https://www.sbmar.com/articles/low-s...arine-diesels/

[nzmal]

Ok so after much messing about I had a friend dive on the boat and measure things. There is only 15mm clearance above the 16 inch prop so that rules out a 17 inch...
I'll talk to A prop guy this week and determine options from there.

[Scout 30]

Originally Posted by nzmal

Ok so after much messing about I had a friend dive on the boat and measure things. There is only 15mm clearance above the 16 inch prop so that rules out a 17 inch...
I'll talk to A prop guy this week and determine options from there.

The minimum clearance for a prop is 15% of the prop's diameter. For a 16" prop 61 mm or 2.4 inches with 2" the minimum for any prop. With the blades too close to the hull you get increased noise & vibration from the water hitting the hull & I've read that the water can actually abrade the hull. That means that you should be looking at a 15" prop. That being said, if a 16 has worked for you for a long time & there's no hull damage ...

[Cadence]

Originally Posted by Scout 30

The minimum clearance for a prop is 15% of the prop's diameter. For a 16" prop 61 mm or 2.4 inches with 2" the minimum for any prop. With the blades too close to the hull you get increased noise & vibration from the water hitting the hull & I've read that the water can actually abrade the hull. That means that you should be looking at a 15" prop. That being said, if a 16 has worked for you for a long time & there's no hull damage ...

Thanks for putting a # to my post 16.. I could not remember what it should be in order not to lose efficiency.

[Scout 30]

Originally Posted by Cadence

Thanks for putting a # to my post 16.. I could not remember what it should be in order not to lose efficiency.

This is not about efficiency. It's about water moved by the prop hitting the hull. The larger diameter prop will always be more efficient.

[Cadence]

Originally Posted by Scout 30

This is not about efficiency. It's about water moved by the prop hitting the hull. The larger diameter prop will always be more efficient.

Not if it's creating turbulence from being to close to the hull or skeg. The power developed starts at the outer edge. In free water yes. I'd rather have the biggest slowest prop. I could have for a displacement hull. I believe we are attempting to say the same thing. An oversized prop. for the aperture is not the way to go.