Need help with repower issues

Hello group,

I have a commercially marimized Kubota diesel which I purchased bobtail.
I need to get the transmission and prop details in order so I can get the old girl back in the water.
The particulars:
20,000 lbs sailboat
28.3' WL 41' overall
10.3 beam
full keel cutaway forefoot attached rudder underbody.
engine- Kubota 35.5 hp at 3000 rpm (gross intermittent)

I am hung up on several issues
transmission:
I purchased and have bolted on a borg warner 71c 1.91:1
I wish to match the engine/trans with an autoprop

Seems maybe I should have gotten a 2.10:1 transmission so I can swing a larger wheel;
16" vs 17" as I work it out....
Hydraulic transmissions I now understand have a lower transmission efficiency than a gearbox. I need a in-line transmission but would like a gearbox if It could buy me a bit more hp to prop/greater efficiency.

Questions:

Is the ratio issue of merit? The boat tends to pitch into a sea and I would like to design in power to punch through.

Is the few percent loss in the hydraulic transmission a real issue? I would like to get all the range I can. Is there a well known in-line gearbox for a small diesel?

Thanks much

 

"Hydraulic transmissions I now understand have a lower transmission efficiency than manual gearboxes. I need a in-line transmission but would like a manual box if It could buy me a bit more hp to prop/greater efficiency."

A hyd tranny in a boat is NOT like a hyd or auto tranny in an auto.The "loss" is simply from heating some extra oil with gear drag.

Mostly the hyd is to make it very easy to shift , so a simple push pull cable works , rather than a more difficult to install mechanical shifting.

You will never notice the difference.

Although there is a "book " difference in the thrust from a 16 or 17 inch prop
much more of the boats performance will be detirmined by all the other factors.
Exhaust restriction , fuel quality , pitch selection, bottom condition, et all.

Your boat will experience no computer) run at 6.5K at cruise (1800rpm) running about 3/4 gallon per hour.
How much it will slow down attempting to best severe head seas will be detirmined by the named factors.

FAST FRED

 

I saw this thread about 5 minutes after you posted it. I have been thinking about it this afternoon whilst walking around the supermarket. Its a shame you didnt post this enquiry before you bought the engine and gearbox. I am sorry to tell you that you wont be doing much "punching' through seas with 35HP in a 41 footer. In just 20Kts on the nose my guess is you will be going backwards.
So as to not seem so negative and have my rep points eaten away. look at other boats and sea how much Hp per foot is normal. For example,-- my friend has a 36 ft Nautuicat with a Volvo 100hp,--it goes well.

 

Hear you on Hp.

The 37 hp is the largest I could fit. The engine is in the bilge and nothing larger will fit. She came with a Atomic four and drives well as she is narrow and draws out to a longer WL at speed. The a-four got us 7 knots in flat conditions and did well against the weather (well used as a relative term) :<). What kills the boat under headsea conditions it the tendency to pitch and stall if the waves are short and steep.... We learned early to motor sail if we really need to go into it & I am looking at the Autoprop for these conditions specifically. One of the reasons I am sweating these details is the addmitably tough job for this installation--- I would like to get it all in good order including the exhaust details and power run aft.

T

 

Re: Autoprop

There are a several different manufactures of the auto feathering props. One or two of them can adjust the pitch while in the water. Can't remember off the top of my head which are which. Most of the companies do a pretty good job of getting the right diameter and the pitch close if you give them your boat parameters. An "in the water" adjustable prop can be fine tuned much easier.