Help choosing a prop for my b20

[Rambling Man]

Hello All,

I just finished restoring a 1968 20 foot bahia mar. It has an old 190 HP mercruiser 4 banger. Any ideas where to start with a prop selection?

thanks in advance.

[mike ohlstein]

Welcome to the Bertram family.

I'm sure that someone will be able to help you.

[Navatech]

I just finished restoring a 1968 20 foot bahia mar. It has an old 190 HP mercruiser 4 banger. Any ideas where to start with a prop selection?

Welcome to the sandbox!... As for your question, your best bet would be to get an answer from somebody who has that boat with that drive... Your second best bet would be one of the numerous "prop calculators"... But, even with a prop calculator it's going to be a bit of trial and error process... See if you can borrow used prop(s) for the testing... Most prop shops should be able to help you out with both the advise and the loaners...

[Charlie J]

therse a guy right around the corner from me that just re did a 20 Bahia mar
if I see him I will ask

[Howesounder]

If you want to try the prop calculator search Victoria prop calculator on Google, and try theirs. I have found it much better than many I tried on ease of use. I used it on a number of projects, and it was bang on, or close enough that adding or subtracting a bit of cup got me there. The last one was my 25 express with a single 150 outboard, and it was bang on. 14 inch pitch got me to 6000 rpm redline at wide open. Also depending on where you buy your prop, some outfits up here will let you try, and exchange a prop if you keep it pristine, so perhaps not getting the best deal on a prop in this case can be worth it if they allow exchanges.

Steve

[Tony Meola]

Maybe if Rocket or Ian see this post they can help out. They have 20.

I think John had one also.

[John Swick]

I haven't gotten to that project yet.
However, I believe 14 x 21 3 blade would be a good starting point.

[ianupton]

My Bahia has a 4.3 225 hp Volvo Duoprop with a 2.32 out drive. Currently have FH6 props and am underpropped. I just got a set of FH8 that should do the trick.

Also know of another with a 4.3 220 hp mercruiser bravo III. It has a 2.43 out drive and a 28 inch prophet.

The merchant set-up hits 50 MPH and I expect mine to as well once the new props are installed. That might help figuring out the "hull factor" in the calculators.

Ian.

[bertram 20]

i had a 68 bahia mar with a mercruiser 350 and we ran a 21 pitch prop and if i had a lot of extra weight i would change it to a 19p prop to get it up on plane better

[ianupton]

Update on the FH8 propset.

Wow... all smiles... still turns up to a full 4920 rpm with a light load but full fuel (110 gallons).

Will have some more weight in her tomorrow, but I think the FH8s are perfect.

Top end increased from 45 to 50 mph.

That is approaching if not reaching hull speed.

Ian.

[Navatech]

That is approaching if not reaching hull speed.

Ian, Im not sure what you mean by "hull speed" but I'm pretty sure you're using the wrong term... The term "hull speed" applies to the top speed a hull can do while in "displacement" mode... For a 20' vessel the "hull speed" would be 5-6 knots... Our Bertrams are known as "planing hulls" and bigger Bertrams (e.g. the 46') are known as semi-displacement, or semi-planing... Which is what allows them to achieve speed that are significantly higher then "hull speed"...

[ianupton]

Nav - I guess I am not sure of the proper term, but every planing hull has a speed that in order to exceed you need excessive amounts of power.

The 20 as an example does about 50 with a 4.3 V6. Put a V8 in there and you might get a few more mph, but no longer anything close to a linear relationship.

I had always heard this referred to as hull speed.

Ian.

[Navatech]

Nav - I guess I am not sure of the proper term, but every planing hull has a speed that in order to exceed you need excessive amounts of power.

The 20 as an example does about 50 with a 4.3 V6. Put a V8 in there and you might get a few more mph, but no longer anything close to a linear relationship.

I had always heard this referred to as hull speed.

Displacement hulls actually do have a top speed above which it doesn't matter how much HP you throw at it, the speed increase will be negligible... Planing hulls and semi planing hulls use the extra HP in order to "get out of the hole" (i.e. plane)... Basically the hull is shaped so that above "hull speed" the water pushed by the vessel pushes the boat up (and out of the water) thereby reducing drag and increasing speed... The relationship between HP and speed of a planing hull is indeed not linear... Theoretically you could push a planing hull almost completely out of the water (look at offshore racing) and every HP will push the boat faster...

[Rambling Man]

Any updates from anyone with a b20? I have tried the calculator and I think a 14 inch 19P 3 blade would be a good start, but I would prefer to hear from those with experience. It is finally getting near time to splash the old girl.

[ianupton]

My B20 with a Volvo 225 hp V6 duoprop.

Ratio: 2.32
Proset: FH8

Another B20 Bahia with a Mercuiser 220 HP Bravo III
Ratio: 2.43
28 inch prop set.

Engine and drive ratio are obviously factors.