OEM prop on a PT-38

[Ed]

I'm trying to purchase a new prop for my Performance Trawler 38' but don't have any record of how the boat was propped from the factory.* The power plant is a Perkins*T6.354, 185 HP with 2.5:1 reduction gears.* Does anyone have access to the original prop size?* Thanks.

Ed
Poulsbo, WA

 

[FF]

If no one can come up with the numbers , give the weight and best guess at what the old top speed was claimed to be and I'll run it thru the computer.

If you have a desired cruise speed , or fuel flow at cruise state it.

FF

 

[Ed]

Thanks, FF.* I'm working with a prop shop that is running the numbers in the computer.*

I just completed a yard period that included bottom paint and prop tuning/balancing.* After refloating the boat a*WOT run produced 9.6 kts. @ 2200 PRM and an overheating situation.* The Perkins would not do the expected 2400 RPM.

I'm trying to compare the 3 blade 25x25.6 prop that was on the boat when I bought it with OEM info to see if an overload situation is causing the overheating.* The other thing I'm doing is using a digital tach to verify the accuracy of the onboard tachs.

Ed

 

[Marin]

Ed---

One thing to keep in mind is that some trawlers---- Grand Banks among them--- were typically delivered from the factory with over-pitched props. This was, I assume, to yield good fuel economy at the relatively low rpms these boats were operated at. However these props would not allow the engines to reach their maximum rpm at WOT, and so would load down the engines to a certain degree.

The owners manual for our 1973 GB36 lists the original props as three-bladed, 24" diameter, 18" pitch on one side, 17" pitch on the other. The props that were on our boat when we bought it are four-bladed, 23" diameter with the same 18" and 17" pitches. The engines are rated at 120hp at 2500 rpm. However, we could only get about 2300 at WOT on one side and about 2200 on the other side. The props have just been completely reworked to be 16" pitch on both sides (as well as correcting a number of other problems.)

I don't know anything about the Performance Trawler, but if it's of the same generation of trawler-style boats like GBs, CHBs, etc., it's possible the original props were over-pitched to start with.

 

[FF]

"Overproping" for most vessels run at displacement speeds is a GREAT idea.

With a Flank of under 10K , the guess is you will cruise at SL (square rt of lwl) times 1.1 for good long range and low fuel use.

This is usually 2 to 4 gph , or 36 to 70 hp at the prop , a long way down from the WOT rating.

FF

 

[Ed]

Marin,

Oh what i would give for a 1983 PT38*owner's manual.*

Looking at the existing prop, it could well be the original.* No bets either way.*

My initial WOT run only netted 2200 RPM.* That prompted my prop shop to loan me their digital tach to check the on board tachs and the transmission ratio.* Interestingly enough when we took the boat out today for further testing it ran up to 2335 RPM.*

An inspection of the maintenance log indicates the primary engine filters haven't been changed in 10+ years.* Why?* Because the Racors only showed 1.5" of vac.* (They weren't changed either).* I'm going to change all the fuel filters and then retest to see if there is a further improvement in WOT RPMs.

Ain't learning a new boat fun?*

Ed

 

[Marin]

Ed---

I'd originally suggested in this post that you have a diver check for numbers on the prop but I see in a reply on T&T you've already examined the prop for numbers as well as had it scanned.

Regarding an owners manual, while you may have already thought of this, I see there are several PT 38s for sale on the web. It might be worth a phone call or two to see if any of these boats have their original owners manuals on board. If so, perhaps the seller or selling broker might be willing to see if the manuals contain the specs on the factory props.* They might even be willing to copy the whole manual for you.

-- Edited by Marin at 02:17, 2008-03-10

 

[Chris Foster]

Marin -

What's the result of the repitch been in terms of performance, etc???

 

[Marin]

Well, we've only had the boat out for a run round the bay since it went back into the water so our observations at this point are based on impressions, not actual data. But I would say in terms of performance there is no discernible difference at all. Boat speed through the water at our cruise rpm settings is basically the same on the knotmeter as it was before the props were reworked. Idle-speed maneuvering is unchanged in terms of how fast the stern can be walked one direction or the other. Idle speed through the marina is exactly the same as it was before.

We observed only two differences as a result of having the props completely reworked and repitched. The boat now tracks much straighter much longer when the wheel is released. Before, the boat would start moving off to port fairly soon after releasing the wheel even though we never had to hold any discernible pressure to hold a course. Where before the king spoke would be at 1:00 when we were holding a course, it is now just a tiny bit to the left of 12:00. Given what the prop shop told us about how horribly the props had been set up before, they were clearly generating a degree of asymmetric thrust.* Or perhaps turbulence that was acting against the rudders although there was no evidence of this at the wheel.* Our boat has mechanical cable/chain steering.

The other difference, but I haven't had time to really study it, is the EGT readings seem to be a tiny bit lower for a given rpm than they were before.

There are some audible differences (improvements) which I assume are due to the complete re-working of the props, the installation of a brand new shaft and cutless bearings on the starboard side, and the installation of a much larger and more positive shaft coupler to match the one that was installed on the port side two years ago.

So I would say we spent a fair amount of money to get things mechanically to where they are supposed to be which should lead to longer lives of the components. But in terms of actual performance change, it's so subtle it's virtually unnoticeable at the power settings we normally cruise at.

The props were not repitched all that much anyway. From 17" and 18" to 16" each. The main thing is we got rid of the mismatched blades, mismatched blade angles, variation in pitch between blades, and a warped hub. And the props are correctly balanced now.



-- Edited by Marin at 01:48, 2008-03-11

 

[FF]

the EGT readings seem to be a tiny bit lower for a given rpm than they were before.

Taking pitch out of a prop lowers the work it is doing , so a lower EGT (at same rpm) would be normal.

 

[Marin]

FF-- I realize that, which is why I mentioned it as a possible benefit. However we didn't really have much time to study it or compare readings at higher power settings. I will be surprised if our initial observation of slightly lower readings at our cruise rpm settings prove to be valid, however, since at those power settings the engines are not working very hard anyway, regardless of what the props were like before or after they were re-worked. I would expect to see it at the high end of the power band, but not down in the middle or lower.