Grew 237 Prop Question

[dklortie]

Hi All,
I am hoping that someone on this forum is able to help me.

This spring we bought a 1986 Grew 237, 5.7L 260hp with 4 barrel Quadra-Jet carb and an Alpha 1 (1.47:1 gear) outdrive and a 3 blade propeller for use as a search and rescue boat on Georgian Bay.
I am thinking that I have the wrong propeller on the boat.
Currently WOT is 4600 RPM (so right in the middle of the WOT spec for the engine). However, I can only get about 27 knots on flat water and that is with it trimmed just right according to my GPS. At 3500 RPM, boat cruises at 21.2 knots and if I bring it back to 3200 RPM (the supposed best economy for this engine), it will fall off plane even with the trim tabs.
I have no idea what pitch the propeller that is currently on the boat is as there are no size numbers on the outside of the propeller, and the numbers that are cast on the inside of the hub don't seem to match up to anything on doing an internet search.

I have a friend that has the Grew 230 Scorpion (same hull as mine) and he can cruise at 3200 rpm at 27-30 knots, and pushes over 40 knots at WOT.
I know that my boat is a little heavier as it has the head and small galley in it, but should still be close to the same performance numbers I would figure.

I am wondering if there is anyone that has this same model of boat on the forum that can tell me what they are using for a propeller size and what kind of performance numbers I should expect from the boat. I can't seem to find any information on this on the net.

When I use the calculators on the web for propeller slip, assuming a 19 pitch prop, it shows that it is 44% slip which I figure is way too much and what is causing the sluggishness, but that doesn't explain why I am right in the middle of the RPM WOT range which would suggest the proper pitch? A 21 pitch in the same calculator shows a 48% slip!

I am hoping that someone can point me in the right direction to where I don't have to spend a couple thousand dollars on a selection of props to figure out what I need.

Regards,
Darren

 

[QBhoy]

Hi there.
I think your rpms and speed aren’t to bad to be honest. And your reported cruising speed and rpms aren’t unusual either.
I would also say that there isn’t a text book best economy rpm that stands true for every boat with the same engine. The best economy for a boat is not so much about rpm, but rather what’s best for your boat individually. So usually the most efficient state is when the boat is on plane and happy on plane. As in sitting on full plane and not wanting to fall off the plane.
At the risk of boring you…I always followed the general rule that best efficiency was when on plane and no more and about early 20’s mph. But having a look at my fuel and range calculator on the smart craft on my mercruiser…it’s actually more efficient when going a little faster and about late 20’s. So basically, every boat is different in what speed it’s happiest at. You’ll usually know when your boat is happy and sitting at a happy speed. Usually around early to mid 20’s (mph) I’d say, in general.
I’d also suggest that there is no way you have a 19” propeller on your boat. If it’s a 1.47 ratio. It will be much much less than that. More likely around 15/16”, I’d guess and no more than 17” for sure. Can be certain of that.

 

[dklortie]

Hi there.
I think your rpms and speed aren’t to bad to be honest. And your reported cruising speed and rpms aren’t unusual either.
I would also say that there isn’t a text book best economy rpm that stands true for every boat with the same engine. The best economy for a boat is not so much about rpm, but rather what’s best for your boat individually. So usually the most efficient state is when the boat is on plane and happy on plane. As in sitting on full plane and not wanting to fall off the plane.
At the risk of boring you…I always followed the general rule that best efficiency was when on plane and no more and about early 20’s mph. But having a look at my fuel and range calculator on the smart craft on my mercruiser…it’s actually more efficient when going a little faster and about late 20’s. So basically, every boat is different in what speed it’s happiest at. You’ll usually know when your boat is happy and sitting at a happy speed. Usually around early to mid 20’s (mph) I’d say, in general.
I’d also suggest that there is no way you have a 19” propeller on your boat. If it’s a 1.47 ratio. It will be much much less than that. More likely around 15/16”, I’d guess and no more than 17” for sure. Can be certain of that.

Thanks for the information. Circumstances have prevented me from trying out different propellers to date. Over the weekend I did confirm that the propeller on the boat is a 15 x 17 Quicksilver 3 blade. With 4 people on board, 4000 RPM is required to get the boat up to 21 knots, and takes about 30 seconds to get to that speed.

I have an opportunity to try a 14 x 17 4 blade. In your opinion, would that help with the prop slip, going to a smaller diameter but keeping the same pitch and adding a 4th blade?

 

[QBhoy]

Thanks for the information. Circumstances have prevented me from trying out different propellers to date. Over the weekend I did confirm that the propeller on the boat is a 15 x 17 Quicksilver 3 blade. With 4 people on board, 4000 RPM is required to get the boat up to 21 knots, and takes about 30 seconds to get to that speed.

I have an opportunity to try a 14 x 17 4 blade. In your opinion, would that help with the prop slip, going to a smaller diameter but keeping the same pitch and adding a 4th blade?

More blades equals more prop slip usually.
So back to the rpm…was the 4000 rpm as much as you could get ?

 

[dklortie]

More blades equals more prop slip usually.
So back to the rpm…was the 4000 rpm as much as you could get ?

WOT is still 4600. That is what is confusing me as it is right in the middle of the 350's recommended WOT range. However, the boat acts like a slug in the water. Hole shot takes forever with 4 on board, and to get 21 knots out of her required 4000 RPM, and that was on flat water. However, when you look back at the rooster tail, you can see that there is a LOT of prop wash moving backward beyond the rooster tail which is about 8 feet behind the boat. Like what it would be when you are throttling up and trying to get on plane and pushing water backwards.

 

[QBhoy]

WOT is still 4600. That is what is confusing me as it is right in the middle of the 350's recommended WOT range. However, the boat acts like a slug in the water. Hole shot takes forever with 4 on board, and to get 21 knots out of her required 4000 RPM, and that was on flat water. However, when you look back at the rooster tail, you can see that there is a LOT of prop wash moving backward beyond the rooster tail which is about 8 feet behind the boat. Like what it would be when you are throttling up and trying to get on plane and pushing water backwards.

You think you’re trim is ok ? Like the drive position trim when coming on plane then to suit later when running flat out ? I’d say on that set up. About 3500 should have you planing and cruising

 

[dklortie]

You think you’re trim is ok ? Like the drive position trim when coming on plane then to suit later when running flat out ? I’d say on that set up. About 3500 should have you planing and cruising

That is what I was thinking, 34-3500 should be where I need to be. Drive is trimmed right down at throttle down for getting on plane. I have to raise the drive trim as the boat starts to accelerate even before on plane in bursts or it will not even get out of the hole. Once on plane, I have tried trimming up to the point that it starts to cavitate, and then drop it back down slightly until I don't see any prop wash coming over the rooster tail (should be the best trim to get thrust straight back). It all has me totally baffled. If my WOT RPM was off I would know what to do, up or down in pitch, however it is right in the middle of the RPM band so ..........

 

[racerone]

Have motor performance checked on a dynamometer.-----Have you checked the boat weight?-----Could be water logged foam if it has foam floatation under the floor..-----Where abouts on Georgian Bay are you doing / volunteering search and rescue with a 35 year old boat ?

 

[dklortie]

Have motor performance checked on a dynamometer.-----Have you checked the boat weight?-----Could be water logged foam if it has foam floatation under the floor..-----Where abouts on Georgian Bay are you doing / volunteering search and rescue with a 35 year old boat ?

Southern Georgian Bay (Severn Sound) area. Boat may be a 35 year old boat, but Grew boats were built in Penetanguishene for Georgian Bay waters and boat is in perfect shape for it's age. Engine only has 650 hours on it.. The boat that I was crewing on before they retired was a 1978 Grew 255. It is part of the Coast Guard Auxiliary, which is more or less a owner/operator volunteer organization assisting in marine search and rescue missions utilizing privately owned vessels. Would I like to have a new 30' enclosed workboat like a Stanly or such, absolutely, however, reality is there is nothing wrong with the boat other than it has way too much prop slip right now. She is good and solid and a great big water boat.

As far as the foam is concerned, I am thinking that might be part of the problem. The boat is definitely bow heavy. There is no foam from the forward bulkhead back as I can see into the bilge through the lockers right up to the forward bulkhead. Only place is under the floor in the cuddy, there is a sub floor in the bilge that is covered over and glassed to the hull. I think that this may be where the foam is.

 

[mvn]

I had a 1970 Grew 234. One of the first projects undertaken before launching it was to cut the cuddy floor out and remove the waterlogged foam. Best I could tell, water came in through the rub rail screws.

Mark