T&T: Propeller Pitch

[Jan Ölander]

I’m a happy owner of a Marine Trader 34 Europa, but I have an issue with the
speed. According to documentations the top speed is 10.5 knots, but mine is
hardly doing 9 knot “flat out” at 2400 rpm. We normally cruise around 7
knots so the top speed is not a real problem, I just wondering if I have the
right propeller.
My prop is a 23 LH 16 and I think that the pitch is maybe too small. Does
anyone out there have a similar boat, and do you know what pitch you got? I
have a Ford Lehman SP135 and Velvet Drive with a 1:2 ratio.
Jan Ölander
MT 34 Europa, Easy Livin’
Västerås, Sweden
http://tinyurl.com/nd253qe

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[Steve Sipe via Trawlers-and-Trawlering]

Jan,

The correct pitch for your prop will be indicated by how close you are
to the engine's rated rpm @ WOT (wide open throttle), not the speed
you're achieving. The objective is proper engine loading, speed is
simply a product of throttle and bottom condition. Although you can
influence your cruise speed at a given rpm by adjusting the pitch of the
prop, that's not the right way to go about it. The pitch should be
corrected to yield the rated rpm@WOT. If you want to increase your
cruise speed, push the throttle up. Even though you may have to run at a
higher rpm to get the speed you'd like, your engine will be running at
reduced load with the end result of improved economy. Overloading will
cause sooting, overheating, and is just hard on the engine, particularly
over a long period of time.

I don't know the rated WOT for the Lehman, I'm sure there are plenty of
folks on the list who have those engines and can provide that information.

On 2/7/2015 7:35 AM, Jan Ölander via Trawlers-and-Trawlering wrote:
> I’m a happy owner of a Marine Trader 34 Europa, but I have an issue with the

> <snip>
>
> --
> Steve Sipe
> Solo 4303 "Maerin"
> Ft. Lauderdale

[Bayside Marine Surveying, Inc. via Trawlers-and-Trawlering]

Hello Jan

According to the information that I have, your Lehman 135 wide open throttle (WOT) is 2600 rpm. Check the WOT rpm at the engine crankshaft with a photo or other accurate direct reading tachometer. Installed gauges are notoriously inaccurate.
If the RPMS are still low then check the usual issues first, bottom condition, propeller condition, fuel, air and overall engine condition, If all else is proper then it sounds like you have too much pitch if the engine only turns 2400 WOT. By reducing pitch you may lose some top end speed but the engine will not be loaded as much which may affect fuel burn downward.

According to the information that you provided and utilizing a propeller calculator, if you have a 3 blade 23" prop the pitch should be about 14.5". However, I dont have the weight of your vessel and this is only a guide. My experience from these boats that I have surveyed with the 120 or 135 Lehmans that WOT speed is usually 10 kts or less with a 6 - 8 kts cruise for optimal fuel useage. I hope you find this information useful.

Alan C. Gaidelis, AMS #784
Accredited Marine Surveyor
Bayside Marine Surveying, Inc.
Middle River, Maryland
www.baysidemarinesurveying.com

[Jan Ölander]

Thank you Alan for your clarification.

Jan

-----Ursprungligt meddelande-----
Från: Bayside Marine Surveying, Inc. [mailto:yachtin...@aol.com]
Skickat: den 7 februari 2015 15:49
Till: jan.o...@telia.com; traw...@lists.trawlering.com
Ämne: Re: T&T: Propeller Pitch

[Ken Bloomfield via Trawlers-and-Trawlering]

Hello Jan,
Your boat looks very nice on your website. It looks a lot like mine,
which is a 50' MT Walkaround. You have received a lot of info on WOT
versus speed, but I will make a more basic comment. That is that I
think it is a dream to get 10.5 knot speed out of it. I could with a
small stretch believe 10.5 statute miles per hour which would be
10.5/1.15 = 9.13 knot, but not much more.

Also, in all the advise that you get about achieving the full rated RPM
on your diesel at WOT being an indication that your prop is correct,
there is a caveat. That is that if the engine was a bit sickly, and
someone found that it did not reach full rated RPM at WOT and then
resized the prop rather than fix the engine issue, then you can easily
reach rate RPM at WOT and still be propped wrong. My bet is that you
are propped just fine, and your 9 knot speed is just what you should expect.

Ken Bloomfield
m/v Tellico Lady.
www.tellicolady.com

On 2/7/2015 7:35 AM, Jan Ölander wrote:
> I’m a happy owner of a Marine Trader 34 Europa, but I have an issue with the

> speed. According to documentations the top speed is 10.5 knots, but mine is
> hardly doing 9 knot “flat out” at 2400 rpm. We normally cruise around 7
> knots so the top speed is not a real problem, I just wondering if I have the
> right propeller.
> My prop is a 23 LH 16 and I think that the pitch is maybe too small. Does
> anyone out there have a similar boat, and do you know what pitch you got? I
> have a Ford Lehman SP135 and Velvet Drive with a 1:2 ratio.
> Jan Ölander
> MT 34 Europa, Easy Livin’
> Västerås, Sweden

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[john mccarley via Trawlers-and-Trawlering]

IMHO, manufacturers likely produce their top speed metric with a brand new
boat with non-painted bottom, low fuel loading and none of the water,
electronics, flybridge hardtop, dinghy, tender, house batteries, spares,
etc. you find on a real-world boat like ours. I wouldn't beat myself up for
being 10% off their published top speed.

John
Escape
Wrightsville Beach, NC