Propeller size question

[davarisg]

Hello everyone,

First time posting here. I am constructing a Bertram 31 pretty much from scratch with the bellow specifications:

LOA 31 ft
Beam 11.2 ft
Draft 3.1 ft
1103 lbs / 5t

Single engine (YANMAR 6LPA – STP2)
323KW / 311hp
3800rpm
2.13:1

I am trying to find what would be the right propeller size/type to use. Would greatly appreciate some help.

Thanks in advance!

[Bruce]

You can use the search function and put in "yanmar 6lp props" and come up with all kinds of answers.

[mike ohlstein]

It will never get up on plane....

340 Diesel HP is barely enough to cruise at 17 knots.

420 HP pushes you to 21.

460 gets you to 25.

[Tony Meola]

You are only running a single engine? Unless you are doing major modification that can not be correct. If that is what you are doing Mike is correct, you will need a lot more power and swing a bigger wheel. That will really require major changes.

[Carl]

It will never get up on plane....

340 Diesel HP is barely enough to cruise at 17 knots.

420 HP pushes you to 21.

460 gets you to 25.

I don't know....

The boats came with ol 4-53's at 140hp each that got boat on plane. True they are slow at 17knots WOT. But if single screw the single motor has to be close to half the weight of 4-53 with only one gearbox. Plus half the underwater hardware for less drag.

Then it is not mentioned if it's a sportfish, fbc, Express, sedan, runabout or other of his own design...



I'm not saying it will get up and go...but it would be interesting to see. Gonna be a big draft boat though.



In the club we have a pair of 28' ish Carolina Classics or Albamarles. Both are deisel, one is twin, the other is single. I am not familiar with the details aside from both guys love the performance of their boats...but maybe something to look into to get an idea of what to expect going twin to single on a deep vee hull.


Sorry...I cannot be more helpful aside from asking a good prop guy or finding someone with a similar setup.

[Yannis]

We didn’t ask if it's an I/O or a shaft.
If it’s a shaft, he’ll need a skeg under the centerline, no? So the motor has to be just outside the cabin entrance,...so it's probably an I/O.

[Tony Meola]

Just for reference on planning, I can actually plane my boat on one engine with the 270 Cummins. I would not make a habit of it, I only did it to test to see what she could do. That is a quick way to shorten the life of an engine.

[Carl]

Just for reference on planning, I can actually plane my boat on one engine with the 270 Cummins. I would not make a habit of it, I only did it to test to see what she could do.

Pretty interesting. I had heard it could be done..never really spoke to someone that tried it. Do you recall what RPM and speed she came up on plane? Did you push to see how far RPM's would come up at WOT?
Just using that data one could expect a single 270 diesel would get boat on plane, then consider if power was centered, the ONE motor weighted less then ONE your Cummins...and only ONE motor and trans. Obviously not a speed demon, but anything past 270hp would go into speed gain.

That is a quick way to shorten the life of an engine.

Yes it is...unless the wheels could be modified to make the rated WOT. Of course with two motors you would be under propped...but I'm not sure that is the worst thing in the world either...provided you show the restraint of not going past the rated redline. Guessing and surmising here...if I'm wrong let me know.

While I don't think I'd want a single...It would have to be better on fuel I'd think. Different layout for sure...but then again instead of two big motor boxes on the sides; a single box could be centered. Now as its a Deep V, the motor would be lower...box less intrusive. Access to cabin could be done from the side of box...or sides. Lower center of gravity. Not being an I/O fan I am not even going there...just considering as a straight inboard. Safety factor...is it reduced? maybe some...but from what I've seen most, or maybe many issuers are fuel related, we run off a central tank. Interesting idea...

[Tony Meola]

Carl

That was 10 years ago and if I remember right I backed her off at 2200 RPM's. I did it just to see what would happen.

As far as a single engine, on our boats it creates a host of issues. To try to keep the center of gravity the same, she would have to be right about where the current companion way is. Now you have to change the fuel tank around to two tanks in each of the current shaft alley.

Now less weight in her she sits higher and will roll more. You might be able to compensate with more fuel depending how much the two side tanks will hold. You would need to go to at least 150 gallons per side. Then their is the issue of making sure you draw from both tanks at the same time to keep the boat balanced.

Too many problems to figure out for me.

[Carl]

Tony,
All that single engine design work has been done before on other boats. True it would no longer be a standard 31 Bertram deseign, but we knew that soon as we pulled an engine out. Two motors down low becomes one down lower while the single tank becomes two on the sides. Fuel is leveled with a crossover...old technology.

[mike ohlstein]

Actually, there's this.....

https://www.boattest.com/view-news/3230 ... -two-props

[Carl]

[quote="mike ohlstein"]Actually, there's this.....

While really cool...why bother if you could just go single screw. Seems over complicated and expensive with lots of excess gear and drag for what reason? Maneuverability...toss a bow thruster in.

When I first saw I thought I had seen that before. I did not see it with gears, but I did see with a single motor driving electric motor and motors. I believe that was both efficient and offered infinite adjustibility on shaft/prop speed.
Think about that for a second. Produce lots of torque to get out of the hole and push on plane BUT now the shaft rpm is not limited by motors max rated rpm anymore as you can adjust electric motors rpm directly. So you have higher top speed...then low speed you just dial down the shaft RPM to zero if you want. Ok...one down side is its an electric motor in a salt water environment, but at least it has advantages.

I'll have to search that story...might have been in Boat US or online..

[Tony Meola]

Tony,
All that single engine design work has been done before on other boats. True it would no longer be a standard 31 Bertram deseign, but we knew that soon as we pulled an engine out. Two motors down low becomes one down lower while the single tank becomes two on the sides. Fuel is leveled with a crossover...old technology.

Carl

Yep, knew that, but still too much trouble and still just leads to a lot of other issues.

[davarisg]

Hey guys,

Terribly sorry for the delay in my response. I am installing 2 6LPA's not 1. Sorry about this misunderstanding.

Since I have received a lot of answers concerning the prop size, I wanted to post this question here with you guys. The story is that Michigan told us we should install 21” x 21,25” Nibral, but our engine supplier suggested 20'' x 23.5'' (after supposedly talking to Michigan himself) and ended up importing 20'' x 22''.... Right now I can either order new ones from Michigan which will take about 2 months to get here or install 2 Michigan props I have found in the local market with size 20'' x 24''.

As you can see I've had a lot different size suggestions so I am a bit confused with which one I should go.

Thanks in advance for your interest!

[Tony Meola]

You are close in size. 21 inch is the biggest you can swing. There are those on here that know more than me about this, but you can probably keep what you have and play with the pitch and cup to make it work.

[Carl]

Just giving a bump so maybe someone can offer an answer.

I'd say find someone with similar setup and go for those...but I guess that's why you came here. I'm not familiar with motors to offer any help. If any possibility to borrow a pair of wheels to get you in the ballpark that's always best.

Sorry, not much help other then a bump

[Tommy]

Capt. Pat and Bruce installed the Yanmar 6LP-STE 315 HP engines in my 31 SF in 2000 and I could not have been happier with the results. I see your engines are the 6LPA – STP2, 311 HP, so I cannot speak to the differences in the engine specs. Here's the info on my setup and performance curve:

1973 Bertram 31 SF
Yanmar 6LP STE 315 HP
Hurth ZF Transmissions HSW63OA; Port = 2.10:1, STBD = 2.04:1
Shafts 1.5"
Props 20X23 bronze 4-blade

Sea trial: freshly painted bottom, full fuel (220 gals); 2 people, moderate gear, no tower, wind variable

RPM KNOTS
700 5.1 (Idle)
1200 7.6
2000 13.7
2400 19.0
2600 21.2
2800 22.9
3000 24.9
3200 26.7
3400 28.0
3600 29.6 (Maximum continuous operating RPM per Yanmar)
3800 31.5 (Maximum rated RPM per Yanmar)
4000 33.0 (WOT; "underpropped by design so to still reach 3800 loaded with dirty bottom)

Hope this helps. I would be interested in someone explaining to me the difference in the two different Yanmars. I believe some of the Yanmars were dicontinued due to EPA emissions issues.