Transmission ratio and speed

[Sea Horse]

I have an older 41 C Hatteras with 3126 420hp Cats. I turn 24/24 wheels with a slight cup and cruise at 21/22 knots at 2400rpm topping out exactly where I should at 2850. Spoke to a fellow owner of same boat same engines who claims 24 knots at same 2400 rpm topping out at 2800 . Only difference I see is transmissions . I have 1.75 to 1 he has 1.5 to 1 gears. I also have a big tower he has a hard top. Any experts have thoughts on weather his gears are giving him more speed?

 

[Top Hatt Craig]

Makes sense, your boat has the increased drag (however small) of the tower and a lower gear ratio (higher numerically) enabling higher full throttle RPM. The other boat has less drag allowing a taller gear ratio, that makes for more load on its engines, and slightly more speed. His boat will go a little faster at a given RPM, your boat will go farther between engine overhauls.

 

[Cricket]

The tower is probably costing you 2 knots over just a hardtop boat. To me your boat is propped correctly.

 

[krush]

I have 1.5 and 23x24 4blade with what I feel and would call slight cup (if marked correctly) on 2inch shaft and I need to re-check, but I'm about 24kt at 2400. Cummins 8.3 430hp. Topped out in the fall at 29.5 (no tabs). No tower.

 

[Boatsb]

That's a big and heavy tower. Plus all the other weight added ( fuel and gear) in the cockpit.
I believe you cary 200 gallons more than most.

 

[krush]

Are the engines painted yellow? That could be the problem!

 

[dsharp]

A 1.5 gear would be a big negative. The difference in speed is due to the tower . The drag is significant

 

[krush]

A 1.5 gear would be a big negative. The difference in speed is due to the tower . The drag is significant

That's what I thought, but I'm running faster.

 

[Sea Horse]

That's a big and heavy tower. Plus all the other weight added ( fuel and gear) in the cockpit.I believe you cary 200 gallons more than most.

Scott I carry 600 gal but I'm using 300 gal for purpose of my question.

 

[krush]

An old fart told me that 46c (I think??) came with 1.5:1 and 2:1 gears--he said the 1.5 ran a tiny bit faster, all else being equal.

 

[Boatsb]

Scott I carry 600 gal but I'm using 300 gal for purpose of my question.

But the weight of the tanks, fishbox, tower and other items makes the boat heavier. I know because I ran it and have a 1966 41c myself.

The transmissions are right for the setup. Trying to squeeze an extra kt or two is nice but remember it's not a stock setup and neither is krushes.

I'm working on a repower quote for a 42 ocean and the transmissions will need to be 2:1 to accommodate the 1.75 shafts. However at 1.5 the software has slightly better performance in theory.

I believe the faster turning props help.

 

[krush]

In theory, a larger, slower turning prop is more efficient. But things really get out of theory land and into empirical land when dealing with planning vessels. Real world evidence always wins in this case.

 

[Boatsb]

I have 2700 rpm 280 hp Detroit's and a 1.9:1 transmission.

My boat is 2 kts faster with 3 blades.

Riddle me that.

 

[jim rosenthal]

Isn't it true that when you're doing a repower, the calculations and all get you close (you hope) but then you may have to try several different prop sizes to get zeroed in on the best for that particular boat?

 

[Boatsb]

That's 100% true.

The calculations deal with expected loads and forces to determine shaft size and transmission ratio. It calculates prop size and load too but there's always a bit of fine tuning at the end.