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Old 04-18-2019, 11:27 AM   #1
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Somewhat unique Prop question ?

Hi A'll, I have a 78 Mainship 34 MK1. It has been repowered with a 454 sometime ago (around 800 hours) still runs great. The question I have is that it has a 3 blade prop. I just took it off for the shop to clean and balance etc. forgot to measure it. What should or would you suggest to be in this boat but with the 454? I run about 3000-3200 RPM at about 8 knots right now.
I just have no comparison as to what things should be or could be..again the 454 throws a liitle monkey wrench in the thought process here.

Open to opinions, discussion etc. as I feel we have enough to work with for some debating and potential improvement with a new prop etc. Thanks in advance. -Dave
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Old 04-18-2019, 11:32 AM   #2
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What is a 454, a gasser?


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Old 04-18-2019, 12:10 PM   #3
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What is a 454, a gasser?


David
Yes. Big block Chevy.
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Old 04-18-2019, 12:34 PM   #4
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It is going to be very difficult to make any recommendations since yours may be the only 34 Mainship with a big block or any gas engine. I am surprised that you turn 3000+ rpms to get 8 knots. I would have guessed you would be going significantly faster, but I am used to planning hulls with big blocks. Most planning hulls at 3000+ rpms you would be on plane. Interesting...
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Old 04-18-2019, 12:44 PM   #5
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It is going to be very difficult to make any recommendations since yours may be the only 34 Mainship with a big block or any gas engine. I am surprised that you turn 3000+ rpms to get 8 knots. I would have guessed you would be going significantly faster, but I am used to planning hulls with big blocks. Most planning hulls at 3000+ rpms you would be on plane. Interesting...
I hear you, why I figured best to ask this group. Also yes most are planing at 3000. Of course being a semi Displacement Iíll never plane
Wondering if I can change up the prop for some better numbers fully knowing and not looking for speed per day but more efficient or better power out of the hole for docking etc are seemingly reasonable things to chase despite the hull and weight .
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Old 04-18-2019, 12:48 PM   #6
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For the prop shop...
You would need to know the trans ratio, the prop size (D XP) and what your max rpm is with the existing prop as a test with the boat configured for normal use.
The prop[ shop has your prop numbers since they have your prop - the rest is up to you.
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Old 04-18-2019, 12:51 PM   #7
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I hear you, why I figured best to ask this group. Also yes most are planing at 3000. Of course being a semi Displacement Iíll never plane
Wondering if I can change up the prop for some better numbers fully knowing and not looking for speed per day but more efficient or better power out of the hole for docking etc are seemingly reasonable things to chase despite the hull and weight .
"Wondering if I can change up the prop for some better numbers"
So are you having the prop cleaned and balanced now or not?

"better numbers fully knowing and not looking for speed per day but more efficient or better power"
What are your baseline numbers now? what are you trying to improve?

FWIW - our past 34 Mainship with a single diesel had a 20 x 20 Nibral prop suited for the lower rpm and hp diesel. Those numbers will not apply to your application.
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Old 04-18-2019, 01:02 PM   #8
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Boatdiesel.com has a prop calculator that may or may not be useful to you. I am not sure if their calculator would apply to gas engines. But you will need the info mentioned by others about prop size, trans ratio, waterline length etc. it costs $25 to join boatdiesel so you might find another site that has prop calculators for free if you search.
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Old 04-18-2019, 01:06 PM   #9
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I hear you, why I figured best to ask this group. Also yes most are planing at 3000. Of course being a semi Displacement Iíll never plane
Wondering if I can change up the prop for some better numbers fully knowing and not looking for speed per day but more efficient or better power out of the hole for docking etc are seemingly reasonable things to chase despite the hull and weight .
Yes it will get up and "plane". After I repowered my "ex" with a Cummins 6BTA 270 hp it would get up and make almost no wake at 15 knots and above.
I don't know what the max WOT rpm is for a big block chevy, maybe 4400? You need to find out the spec and then document what the boat will do. (with a known accurate tachometer)
If you don't know, 16 knots is about the max "practical" speed the boat will do. After that it "chine walks"(that's what I call it don't want to make nomenclature the topic of this thread) where the boat will rock from side to side until you slow back down under 16.
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Old 04-18-2019, 01:07 PM   #10
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I just had my props rebuilt this winter. I was having a bit of vibration so I thought I would try thr props first. Mine are 4 blade 24X24 props. I had 1/2Ē of pitch taken off them so they are now 24X23.5 props. It cost $980 per prop. But the props are now almost perfect according to the before and after scans. I would take the boat out and get some baseline info as to rpm versus speed so you will know what you have now and can tell waht difference any changes you make provide. Run the boat in 2 directions 180 degrees apart and measure the speed at say every 500 rpms then average the 2 speeds at each rpm to see your actual speed.
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Old 04-18-2019, 01:08 PM   #11
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Do you know the ratio of your transmission?


There are plenty of calculations you can run to estimate slip, required torque and maximum rpm ahead of prop selection to ensure you buy the right prop but since you already have one in place the simplest method is running at WOT and seeing what RPM the motor will achieve. If you are underpropped, the engine will easily redline, if you cannot reach the engine manufacturer's max recommended RPM's you are over propped and should reduce pitch. Those mainships came with 4 blade props so your boat was likely re-propped when it was repowered. Diesels generally run at slower final drive speeds and turn larger diameter props with larger diameter shafts to withstand the torque. When repowering a gaser to diesel, it is often a difficult job because the shaft size needs to be increased and the larger diameter prop may not clear the hull at the angle the original prop shaft was installed which really snowballs into a lot of modification requirements. Going from a diesel to gas is simpler and much cheaper which is why the prior owner did it in the first place. Perhaps his wife couldn't stand the smell of diesel exhaust.
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Old 04-18-2019, 01:11 PM   #12
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"Wondering if I can change up the prop for some better numbers"
So are you having the prop cleaned and balanced now or not?

"better numbers fully knowing and not looking for speed per day but more efficient or better power"
What are your baseline numbers now? what are you trying to improve?

FWIW - our past 34 Mainship with a single diesel had a 20 x 20 Nibral prop suited for the lower rpm and hp diesel. Those numbers will not apply to your application.
Thanks for the replies. Yes they are working the prop now to put back on if nothing else presents itself before splash.
Looking to potentially improve engine efficiency, maybe out of the hole to help move the boat around a little better? All unknowns as my knowledge stops with out drives and one with a Volvo with a DP setup which is all basically useless knowledge for this boat ( owner for 2 years now of the mainship.) I never ran any numbers etc for precious boats as they were all great performers and efficient.
Fully understand I will see no magic bullet here just looking for new knowledge and opinions with this unique power plant situation. Thanks for the info you provided
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Old 04-18-2019, 01:29 PM   #13
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Hey everyone, doing this inbeteeen working and on a phone so just seeing some precious replies.. all good stuff. My buddy just coincidentally dropped the prop off and called me and said it is 20x 15. Again a 3 blade bronze.
Seems like getting this back on and getting these specs while underway is first things first from what Iím reading and makes a lot of sense and what I was leaning towards anyway. Will read in more detail tonight your replies and post then.

Open to what peopleís thoughts are on a 20x15 for this setup?? I can also add knowing the owner it might have been just given to him etc and not spec out like we are trying to do here.. thanks
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Old 04-18-2019, 01:44 PM   #14
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Well, all things considered I can tell you for sure that at 8 knots you're way above hull speed so already getting much worse MPG than you would at lower speeds. The suggestion of trying out the prop and charting the speeds at various rpm would go a long way to figuring out what if anything can be done. At a minimum I would write down speed at 1000rpm and every 500 above that until you either get to max recommended rpm or the engine won't push any faster. To really run any numbers you also need the transmission ratio. It should be marked on the transmission or be part of the model #. Also, if you could find the specs for that motor it could also help determine where the peak torque is.



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Old 04-18-2019, 03:16 PM   #15
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I agree w kchase,
However one would need some fresh water to get good numbers.

I was planing on going to Lake Washington behind Seattle with no tides. Never got around to it though. And as time went by I found many things like fuel burn and speed meant less and less. I still cruise at 2300rpm though. And last I checked it was 6.15 knots.
In the avatar we were at a bit over 6.5 to 6.8. But it matters not as almost never go over 6.4 (2500rpm).
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Old 04-18-2019, 04:05 PM   #16
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Hey everyone, doing this inbeteeen working and on a phone so just seeing some precious replies.. all good stuff. My buddy just coincidentally dropped the prop off and called me and said it is 20x 15. Again a 3 blade bronze.
Seems like getting this back on and getting these specs while underway is first things first from what Iím reading and makes a lot of sense and what I was leaning towards anyway. Will read in more detail tonight your replies and post then.

Open to what peopleís thoughts are on a 20x15 for this setup?? I can also add knowing the owner it might have been just given to him etc and not spec out like we are trying to do here.. thanks
Most of those old Mainships run a 20 x 20 4 blade and a 1.9:1 tranny ratio.
I had a 20 x 21 4 blade with a 1.5:1 ratio after my repower.
I don't know what your tranny ratio is, but if it has the original 1.9 gear, you probably could use more pitch. Those boats can cruise at 9 to 9.5 knots very economically.
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Old 04-19-2019, 10:24 PM   #17
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An average configuration of the 454 is going to develop about 400 Hp at WOT, probably around 4200 rpm. I believe that it is enough to get your boat mostly out of the water. So, 8 kts at 3000 seems slow. I'd expect 8 kts at 1600-2000 rpm. just guessing.
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Old 04-20-2019, 02:29 AM   #18
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vic prop dot com and made some assumptions:
Data Input

Waterline length in feet:31 feetBeam at the waterline in feet:9 feetHull draft in feet (excluding keel):2 feetVessel weight in pounds:16000 lbsEngine Horsepower:350 HPNumber of engines:1Total Engine Horsepower:350 HP Engine R.P.M. (max):4200 RPMGear Ratio:1.9:1Shaft R.P.M. (max):2211 RPM Number of shaft bearings (per shaft):2Desired speed in Knots:16 knotsHorsepower Calculations

This will calculate the maximum horsepower and torque available at the prop(s). Total available horsepower at the engine(s):350 HPTotal available torque ft/lbs at the engine(s): 438 ft/lbsHorsepower loss of 3% per gearbox:- 10.5 HPHorsepower loss of 1.5% per shaft bearing:- 10.5 HP Total horsepower available at the propeller(s):329.0 HPTotal torque ft/lbs available at the propeller(s):782 ft/lbsSpeed & Power Calculations

Basic displacement speed and horsepower requiredDisplacement hull speed (1.34 X sqrt of waterline length):7.46 KnotsMinimum horsepower required at propeller(s) for Hull speed:34.9 HP Calculations based on desired speed and available HPHP required at propeller(s) for desired 16 knots speed:313 HPEstimated maximum speed with existing 350 horsepower:
This is the speed we will use for the propeller size.16.23 Knots At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel. Propeller Size

Number of bladesDiameter (inches) Pitch (inches) 2 Blade21.0X12.5 3 Blade20.0X12.4 4 Blade18.8X12.1 The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.
Website and all contents copyright © Victoria Propeller Ltd. 2015, all rights reserved.
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Old 05-03-2019, 01:20 PM   #19
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No personal experience, but a buddy of mine says his classic Mainship 34 trawler runs a 200 hp diesel and will do 11 knots if he has to, although he's generally running at the trawler speeds of 6 to 8 knots. Seems like alot of wasted HP and gas to be running a big block gas engine in there.
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Old 05-03-2019, 01:59 PM   #20
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No personal experience, but a buddy of mine says his classic Mainship 34 trawler runs a 200 hp diesel and will do 11 knots if he has to, although he's generally running at the trawler speeds of 6 to 8 knots. Seems like alot of wasted HP and gas to be running a big block gas engine in there.

Thanks for the reply. I agree. Got the boat on the cheap with the 454 in it with hopes to put a diesel back in when she goes. Gonna do some RPM and speed tests as instructed in previous replies mid/end of May and report back here.
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