Propulsion, propeller problem

[Rick Hudson]

I have an old 38' trawler and have for some years wondered if I'm under-propped. With a 35' waterline and a 135 HP engine, a 1.3 gear reduction and a 4-blade 24" dia X 20" pitch I should be able to reach hull speed (7.7 knots) at about 1600 rpm, yet even at 2100 rpm I'm not there. Bottom is clean. Stuffing box is fair, as is the one cutless bearing.
Where is the power going, if not into the water?

 

[rslifkin]

What makes you think you should be able to reach hull speed at 1600? What do you get for speed and RPM at WOT? And what is rated WOT RPM on your engine?

 

[psneeld]

I have a 34' waterline.... 22X17 4-bladed prop, 1650 RPM on Lehman 120 gives me 6.3 knots at 3.3 NMPG over nearly 20,000 miles.


I don't think you are underpropped... what is your max RPM?


I get a hull speed of 35' waterline as 1.34x Sq RT 35= 6.85


Note. 1.34 constant varies a bit on hull design but is accepted as the average.

 

[Comodave]

Agree, what RPM are you getting at WOT?

 

[rslifkin]

To add one more question, try double checking your transmission reduction ratio. 1.3:1 seems like not nearly enough reduction for that size prop and I'd expect you to be severely over-propped if that's your actual ratio.

 

[Lepke]

We see these inquiries routinely on more speed from short boats. If you want to defeat hull speed you need to plane or add double or more hp. Or to simply go faster, get a longer boat.
When I was a fisherman I had to settle for 7 knots. I had the power and could go faster but with a lot more fuel. At the time diesel was 35¢/gallon.

 

[Nomad Willy]

Rick,
Your boat in the avatar looks huge.
It’s probably heavy too.
Heavy and beamy = needs tons of power.

Is this boat actually a single?
No mention of twins.

Boat is underpowered.
And many FD boats don’t reach hull speed. The rule of thumb is cruise at one knot below hull speed. I do it. Hull speed of 7 cruise at 6. But to reach hull speed a FD boat needs about 100% power and 50% power to reach cruise speed. And at these speeds your boat needs about half again as much power as a FD.
Unless you have a twin I’m guessing your cruise will be 6.25 to 6.75 knots. In other words you’ve got FD power in a SD or planing hull.

But at any rate you’ll need a prop that will turn about 100rpm over the engine’s rated rpm. A governor kicks in about 300rpm and won’t let you go higher.

 

[High Wire]

1.3:1 ratio sounds strange. Could it be 3:1? A picture of the trans name plate would help.
#2- What type of tach do you have? Most stock alternator driven tachs are notoriously wrong. 10% error is common.

 

[Simi 60]

Cheer men, had to google that up.
Yacht world had some useless info such as..,....

What type of yachts does Cheer Men build?
Cheer Men models vary in size and length from 38 feet to 38 feet

And

How much do Cheer Men boats cost?
Cheer Men boats for sale on YachtWorld are listed for a variety of prices from $69,500 on the relatively more affordable end all the way up to $69,500 for the most expensive, custom yachts. Top-of-the-line models can accommodate motors up to an impressive 0 horsepower, while more economic models may have as little as 0 horsepower engines (although the average power size is 0 HP).

https://www.yachtworld.com/boats-for-sale/make-cheer-men/

The one listed in that link has FL 120 x 2 for 8 knot cruise and 13 at full noise.
Definately an SD hull

 

[Nomad Willy]

Or planing hull Simi.
We ani’t seen no keel yet.

 

[hobbystuff]

Hey Rick:

We used to be neighbors at Westport.

For a data point, our 36' trawler with Ford Lehman 120, 2.57:1 transmission and 24x21.5 pitch 3-blade at 1700 rpm will cruise all day at 7.2 kts. If I wind it up to 2100 I can hit just over 8 kts but I don't enjoy it, although we have done it. I think your boat may be heavier and beamier than mine.

For comparison, I know the rule of thumb for moving from a 3-blade to 4-blade is dropping 1-2 inches in pitch, so in that regard you are comparable to my prop. There is a chance with your setup using a bit deeper gear ratio that you may be a bit light on pitch but it wouldn't be by much.

When I had a new prop made for my boat, I went to VicProp and they basically tuned it for me. Might be worth a shot to ask them, maybe send it in for a tune and repitch based on their knowledge. They took me up 1/2" in pitch and it made a nice difference based on how I like to run the boat.

Cheers.

 

[rslifkin]

Or planing hull Simi.
We ani’t seen no keel yet.

And just to make it weird, some planing hulls have keels (mine included).

 

[Nomad Willy]

rslifkin wrote;
“ And just to make it weird, some planing hulls have keels (mine included)”

Not like a trawler tho eh?

 

[markpierce]

I reach maximum speed of 7.3 knots at 2200 RPM with an engine with maximum RPM of 2400 with four-bladed propeller. Figure I've got a well-balanced boat/engine/propeller.

 

[rslifkin]

rslifkin wrote;
“ And just to make it weird, some planing hulls have keels (mine included)”

Not like a trawler tho eh?

Generally not as deep, but some of the earlier Hatteras designs have a surprising amount of keel down there. The keel adds some drag, but it's only at higher speeds where it starts to become a handling concern. Cruising in the high teens, it's no problem.

 

[High Wire]

New England lobster boats have big keels and plane just fine.

 

[psneeld]

So do old Silverton sport fish/ convertibles. My 37 ran 20 knots all day long at 1NMPG with 2, 3208T Cats. Had around a 2 foot cutaway keel.

Picture is another, not mine.

 

[PM]

I doubt if you are under propped. A 24 inch wheel is a large wheel, the pitch is pretty aggressive also. I don't think you will get much more speed from that boat but it might be worth it to have your prop "scanned" and make sure it is symmetrical, balanced and in proper shape.

pete

 

[Rick Hudson]

Thanks for all the good idea and suggestions. It's comforting that there are others out there with similar systems.

On details, I was today under the boat while on the hard and could read the prop numbers - 1981 (date, I assume) x 24 (dia) x 23 (pitch). Gearbox is not 1 : 1.3 as stated earlier, but 1 : 2.5, which is a big difference. Blame it on a damaged label and old eyes.

Will check no load WOT when back in the water, but manual says 2250. Will also check WOT under load - should be about 2100-2150 for a properly propped boat.

Comments from previous folks are valid - she's broad (13.5' beam) and deep keel (4'), which is one of the reasons we liked her - nothing like a solid keel to protect a single prop in the PNW from log strikes.

Thanks again for input. Much to try.

 

[DavidM]

Will check no load WOT when back in the water, but manual says 2250. Will also check WOT under load - should be about 2100-2150 for a properly propped boat..

I don't think so. More like 2,600 for the Lehman. Check both in neutral (no load) and in gear. If it can't make rated rpm at wot in gear then your bottom or prop is dirty or you have too much prop pitch. If you exceed rated rpm then your prop is under pitched.

David

 

[SoWhat]

I don't think so. More like 2,600 for the Lehman. Check both in neutral (no load) and in gear. If it can't make rated rpm at wot in gear then your bottom or prop is dirty or you have too much prop pitch. If you exceed rated rpm then your prop is under pitched.

I believe he has a Perkins. Sounds like he might be a bit overpitched but first he should use a photo tach to check engine rpm's and transmission output rpm's to make sure there is no slipping.

 

[Nomad Willy]

Rick,
Check rated rpm from the manufacturer and marineizer.
“rated” sounds like some kind of maximum ... it isn’t. It’s a minimum.
You will be reducing pitch to achieve rated rpm.
I like to be 100rpm over. Especially on an application like yours.

You have truck like weight and trucks do well in lower gears. Lower pitch will be your lower gear.

 

[Rick Hudson]

Final word on this - when all else fails, read the manual PROPERLY. It turns out the WOT for the Perkins 6.3544 is not 2250 as stated earlier, but 2800. So, after owning the boat for 13 years, it's like discovering an extra gear in the gearbox of my car! I have been running at 2000 rpm and wondering why I wasn't making hull speed. It transpires I can crank her up to 2600 rpm and reach over 8 knots (> hull speed).

Thanks again for all your input. Together they made me re-check numbers that I had incorrectly assumed years ago. Now, who wants to go waterskiing?

 

[NCheaven]

Prop evaluation

I put my Camano's numbers into Victoria Propeller's prop calculator, and the answer came out remarkably accurate for how my engine and propeller perform. That's a free and easy way to get an independent evaluation starting point. Data: 200 hp turbo, 20x18 four blade prop, 3700 to 3900 rated max WOT, actual recent 3/4 fluid load, actual max SOG both ways of 14.4 kt at 3750 WOT - 2 year old bottom paint, cleaned by diver 2 months before. Model suggested something like 14.7. Model said to achieve 15 kt. I needed 25 more hp. My purpose was to see if I could go to a 20x20 prop and get more speed, but evaluation convinced me to stick with what I have. Will check again after May haul out.

See what Victoria Propeller's model tells you, and determine your next step.

Good luck!