New Propeller Remodeling 38 Chris Craft 1968

[Mikel Arechabaleta]

I have a project of a Chris Craft Commander Flybrige 38 ‘1968.
I bought him 15 years ago in “Trans Atlantic Diesel inc” Dealer , 2 Cummins Recon 6CTA 430 HP engines with ZF gears. model 301A r 2: 1.

I had him disarmed and faced an illness and other setbacks and now that I am getting him ready and I have to do a test. sea trail.

My QUERY: I put shaft 2 “in acuamed 22” and now I don’t know what size?? of propellers I should place?.

can you help me ?

The original factory boat had “federal michigan 23x 26×1 3/8” with 8V53 Detroit Diesel engines.

Topic

[Mikel Arechabaleta]

WOT PROPELLER

So I need to do the sea trial with some borrowed 4-blade propellers (I hope to get whoever has them, or similar) and thus know the performance of my boat in WOT. Then it would be to buy the 4-blade propellers. What is more recommended, and if you think better, which brand? Are used at a good price, where and who could sell them used? I think maybe of having 22 x 22 x 2 to have some idea of ​​costs for the 2 propellers

[Mikel Arechabaleta]

PERFORMANCE TO

you have become an expert in numbers and in navigation, very good, that is very happy. Thank you very much for your help and for the comments, they serve me a lot. I’m going to try that my boat also has its maximum possible performance. Ok so I will place 4 blades on propellers. I will comment on my experience.
Thanks a lot

[pwrobert]

Performance

Well all my boating data is done in MPH. Too much mental gyrations converting back and forth with friends and family when discussing trip data and speed issues when they want to know how far and how long a different trip will be. anyway my chart posted above shows:
Just above idle at 800 RPM gets 7.2 mph (~6kts)
With your boat having an additional 100 HP+ per engine I would feel you should be in the 23-24 kts range for cruise. At 2400 I am at 22.3 mph (19.4 kts).

When I repowered from the 205 HP DD 8.2L engines to the 355 HP 6BTA’s I went from 25 mph top speed to 30 top speed. Went up 2 mph at each cruise setting. But just as importantly went down in burn per hour and from 0.9 miles per gallon to 1.2 miles per gallon all at the slightly faster (2 mph) cruise speeds.

Takes me 9 hours from Florida to the usual first stop at Spanish Cay Ababo Bahamas and around 155-165 gallons for that leg. Depending on sea state of course. All good to to me.

Three blade props usually give a little more top end speed but 4 blades give better load carrying ability and better hole shot. As boat goes up and down seas the additional blade keeps things more steady as the load varies – I feel anyway. Makes your choice. Many feel the CC Commander Series to be “heavy” boats especially compared to today’s boats. I went with the 4 blades to keep the boat able to handle the same when loaded for the Bahamas with 300 gallons fuel all my rods, ice, food and three people geared up for 2-3 weeks.
When the engines are running I am also always running my 8Kw genset.
I have owned this boat since 1990 and done 3 repowers – each time with used engines and can say these 6BTA’s are the best in all respects. I’m sure yours will do you well also.

[Mikel Arechabaleta]

KNOTS AT MINIMUN RPM

And how many knots do you get when engines are at the minimum rpm?

[Mikel Arechabaleta]

KNOTS CRUISE

Thanks

And with those revolutions that you tell us cruising speed? How many knots is the boat making. and how many knots to WOT? I would like for mine about 24 cruise knots and I don’t know WOT … the boat has been a bit heavy for me. I don’t want to burn a lot of fuel

[pwrobert]

Final Result

I normally cruise between 2200-2400 and sometime up to 2550 for short bursts. Mainly for fuel burn or miles per gallon issues. these throttle settings with trim tab adjustments seem to give me the best performance.

I don’t have exhaust pyrometers – just the usual stuff and boost gauges and fuel burn analyzers.

[Mikel Arechabaleta]

Thank you very much for your comment..My boat brought in the original 23 x 26 original propellers. I am going to test the engine with that and in any case your suggestion is very good that I will take into account, and that the WOT max 150 r.p.m more than the 2600 rpm ngine factory max seting. “2750” boat at its maximum weight. how many are your total speed and cruising speed in knot? At How many revolutions per minute?

[pwrobert]

Prop Calculations

I have a 1965 38 CC Flybridge Commander that runs with twin 6BTA 370’s. ZF220A 1.75:1
with Four Blade Nibral 22 X 20 props with heavy cup.

Your prop shop can give you suggested starting point for your prop. Probably in the 22 inch diameter range with some variation in pitch from mine. When I first installed the 6BTA’s the recommended prop was 22 X 22 and I ended up taking out 2 inches to gain the additional RPM’s Tony wanted (3100-3150).

That was two years ago after the repower and it has worked well. Your engines are about the same weight but with more power so with the little extra gear reduction you may be somewhere near mine.

[Author]

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