1978 34' Mainship trawler "propellar woes"

[harry hammett]

Have recently started upgrading a 34' mainship trawler single screw. Having problems getting the hull speed that was originally advertised. I believe a 4 blade 20" x 17 was original. Anybody with thoughts about this. I have no problem with RPM, bottom cleanliness or pitch of hull. It gets to about 8mph at 1700rpm and then just sucks aft downward to 2350RPM.

Thanks

 

[jleonard]

I*owned a 78 Mainship for 14 years. Are you running the original Perkins T6.354 at 160 hp?

Assuming that engine, your OEM propeller should have been a 4 blade 20 x 20.* That would be with either the BW at 1.91 to 1 or 2.10 to 1.* You should be able to reach a minumum of 2450 rpm at WOT (with a calibrated tachometer).

I had a very heavily loaded boat and the 2.1 tranny ratio. I could get about 10.5 knots at WOT (2500 rpm) with the original engine.*

Fairing the keel (by adding fairing blocks ahead of the prop) I gained 250 rpm so then added one inch of pitch to my prop then could get 11 knots at 2550 rpm. Installing Big A$$ trim tabs (42 x 12) will get the arse end of the boat out of the water at 7.5 knots and really help the boat.

If you want to contact me privately try

jleonard@usa.norgren.com during "normal"* working hours (east coast time).

*

 

[jcurry]

jcurry

My 1986 M34 MK III with the 200 hp. Perkins with a clean bottom and a reconditioned 23"x21" 4 blade does 16 knots (gps) @ 2500 rpm.

 

[boatpoker]

Have recently started upgrading a 34' mainship trawler single screw. Having problems getting the hull speed that was originally advertised. I believe a 4 blade 20" x 17 was original. Anybody with thoughts about this. I have no problem with RPM, bottom cleanliness or pitch of hull. It gets to about 8mph at 1700rpm and then just sucks aft downward to 2350RPM.

Thanks

That's about right for hull speed for a 34 footer given the old rule of thumb formula. I'm guessing you're really talking about top speed.

You mention .... 4 blade 20X17 was original but you don't say that's what is on it now ..... so what is on it now ?

What engine, what horsepower ? What gear reduction ?

 

[Baker]

This post is 5 years old.....just FYI

 

[jimisbell]

OK, I can refresh it with my problem. 1978 Mainship 34 with Volvo TDM40A Dont yet know the prop size but the diver said it was 3 blades....not 4 blades like original. But the engine is not original either. Engine tops out at 2800, 8.2 knots, lots of black smoke. Volvo should get to 3600 so obviously over propped. Any idea what prop I need? Will be diving on it again to see if we can read the size and maybe remove it. Hate to pull it out of the water just to get the size.

 

[Lepke]

The size should be stamped in the hub. It should be viewable in the water. Black smoke is usually the engine trying to turn a too big prop, too much pitch, or both. Any good prop manufacturer should be able to recommend a size. Especially with current history.
The TAMD40A is rated at 146 hp @ 3600. In the rating, it is noted the engine should only be run 2 hours out of 12 at the rated hp. Other times at 10% less rpm or 3240 rpm.
If your engine is a TAMD40A, it's turbo charged and running continuously at or near the full hp rating will seriously shorten the engine life. High exhaust gas temperatures cause premature wear of the cylinder components - rings, pistons, cylinder walls and valves. High EGTs are not indicated by the coolant temperature. Commercial boats usually have pyrometers to monitor the exhaust gas temp.

 

[boomerang]

Jimisbell, you didn't mention which clutch/clutch ratio you have coupled to your Volvo repower. That'll play a big part as to what wheel you swing.
I've come to the conclusion that in our 34' with the standard 160HP , 2.10:1 clutch & 20x20 wheel ,we're never going to see much different than an economical low cruise of 7-7 1/2 knots @ 1650, normal cruise of 8-8 1/2@ 1850 & 11-12 WOT 2550.
On another note, it looks like your boat deal went as you had hoped! Congratulations!

 

[jimisbell]

Jimisbell, you didn't mention which clutch/clutch ratio you have coupled to your Volvo repower. That'll play a big part as to what wheel you swing.
I've come to the conclusion that in our 34' with the standard 160HP , 2.10:1 clutch & 20x20 wheel ,we're never going to see much different than an economical low cruise of 7-7 1/2 knots @ 1650, normal cruise of 8-8 1/2@ 1850 & 11-12 WOT 2550.
On another note, it looks like your boat deal went as you had hoped! Congratulations!

The deal isnt signed YET, but unless something nasty shows up at the last minute it will be done by this weekend. The owners are not in a hurry but the lien holder wants it to happen yesterday....LOL. Was going out today to check the engine again but the wind is gusting to 30mph and the bay is white capping so will probably wait till tomorrow.

 

[jimisbell]

The size should be stamped in the hub. It should be viewable in the water. Black smoke is usually the engine trying to turn a too big prop, too much pitch, or both. Any good prop manufacturer should be able to recommend a size. Especially with current history.
The TAMD40A is rated at 146 hp @ 3600. In the rating, it is noted the engine should only be run 2 hours out of 12 at the rated hp. Other times at 10% less rpm or 3240 rpm.
If your engine is a TAMD40A, it's turbo charged and running continuously at or near the full hp rating will seriously shorten the engine life. High exhaust gas temperatures cause premature wear of the cylinder components - rings, pistons, cylinder walls and valves. High EGTs are not indicated by the coolant temperature. Commercial boats usually have pyrometers to monitor the exhaust gas temp.

We ran it for 45 minutes at 2800 and that was its limit. It was smoking at that RPM and topped out at a bit over 8 knots. So, not enough or long enough to cause much wear.

 

[Dwk]

Jimisbell, you didn't mention which clutch/clutch ratio you have coupled to your Volvo repower. That'll play a big part as to what wheel you swing.
I've come to the conclusion that in our 34' with the standard 160HP , 2.10:1 clutch & 20x20 wheel ,we're never going to see much different than an economical low cruise of 7-7 1/2 knots @ 1650, normal cruise of 8-8 1/2@ 1850 & 11-12 WOT 2550.
On another note, it looks like your boat deal went as you had hoped! Congratulations!

That's exactly what we get except the WOT, 2450 and 9.50 -10.50 knots tops. Our wallet and Perkins are happy @ 1850.

 

[FF]

6-5 7K is the ideal cruising speed for a boat with less than 36 ft on the waterline.

If higher speeds are required a different style boat might be called for .

Going from 2GPH to 12 GPH to gain a few K is hard to fathom.

 

[jimisbell]

I agree completely. In my testing I am not looking for higher speeds except to test the condition of the engine. I want to know its in good condition and capable of full performance. I would never......except in dire emergency........ want to waste.fuel like that. Too much Irish blood in my veins......LOL

 

[smitty477]

We ran it for 45 minutes at 2800 and that was its limit. It was smoking at that RPM and topped out at a bit over 8 knots. So, not enough or long enough to cause much wear.

45 minutes with the throttle buried and not achieving rated rpm is one way that you can prematurely have engine failure. Not having EGT gages you would not be aware of conditions of overheating components that can surface later on - the black smoke was a tell tale that it was ongoing.
Please add the EGT gages and/or do not firewall a diesel that has an overload condition.

 

[kapnd]

Ditto Smittys post above.
Once you push the throttle to WFO and observe black smoke and less than full rpm, pull back immediately, knowing that the vessel is over Propped!
Without EGT data, it is very risky to push a diesel like that.
The real start point though is at the dock, run up the engine out of gear to verify that it will make rated rpm. If it won’t make rated rpm and a little more out of gear, there are other problems that must be addressed before you can determine propeller data.

 

[jleonard]

The real start point though is at the dock, run up the engine out of gear to verify that it will make rated rpm. If it won’t make rated rpm and a little more out of gear, there are other problems that must be addressed before you can determine propeller data.

But even prior to the above you need to calibrate the boat's tachometer or else you will be making decisions based on possibly false information.

 

[FF]

"It gets to about 8mph at 1700rpm and then just sucks aft downward to 2350RPM."

This is standard for most displacement boats.

Sounds like the PO installed a cruising prop to increase the engines life and efficiency .
This will give the lowest fuel burn and quietest time cruising.

8 mph is about 7K which is as fast as you would ever push a boat with under 36ft on the waterline.

Use the tach and your GPS every 100 RPM at about 5.5K to find where adding RPM does almost nothing but make the wake bigger.

Most cruisers will run at the sq rt of the LWL times .9 to 1.15 .

Somewhere there you will have a smooth running operation.

Overloading is possible with a too big cruise prop , but in your case if you can heat the ocean at 2300+ RPM operating 500RPM or more down will not be an overload .

When cruising at normal speed add 200-300 rpm of throttle , there should not be a trace of black smoke at the temporary higher RPM, a simple test .

Enjoy , many boats do not share this luxury.

 

[smitty477]

"It gets to about 8mph at 1700rpm and then just sucks aft downward to 2350RPM."

This is standard for most displacement boats.

Sounds like the PO installed a cruising prop to increase the engines life and efficiency .
This will give the lowest fuel burn and quietest time cruising.

8 mph is about 7K which is as fast as you would ever push a boat with under 36ft on the waterline.

Use the tach and your GPS every 100 RPM at about 5.5K to find where adding RPM does almost nothing but make the wake bigger.

Most cruisers will run at the sq rt of the LWL times .9 to 1.15 .

Somewhere there you will have a smooth running operation.

Overloading is possible with a too big cruise prop , but in your case if you can heat the ocean at 2300+ RPM operating 500RPM or more down will not be an overload .

When cruising at normal speed add 200-300 rpm of throttle , there should not be a trace of black smoke at the temporary higher RPM, a simple test .

Enjoy , many boats do not share this luxury.

Your responding to the original post which is 5 years old - I am replying to the updated post made this year.