Surface drives at lower speeds

[Northwester]

I'm considering repower options for my 25' Bertram project boat.

I want to use fixed surface drive technology mostly because it is simple and perhaps is least likely to leave me stranded. To that end, I'm leaning towards installing a ZF Trimax G-drive package.

I would like to use cummins 6BT5.9 210hp marine engine with medium duty rating of 175hp @ 2100 R.P.M. About 420 ft lbs of torque is produced at that same R.P.M. Maximum intermittent hp is 210 @ 2600 R.P.M. The torque curve of the engine seems to be flat.

I would as much as practical position the engine and fuel tanks such that they will work best with surface drive to achieve the optimal running angle. Using some of the performance estimating software avaliable on the internet, the boat might achieve a top speed of 25 knots and cruise at 22 knots with the proposed power plant.

-how well does a surface piercing system work at speeds of 10 - 25 knots?
-is there going to be enough power to get 6000-7000 lbs. of deep V hull onto plane or will more be required?
-propeller selection and reduction ratios which is best for the proposed application and desired performance?
-how far behind the transom does the clean water begin with Hunt deep V hull design?
-how well can hull characteristics be simulated/modelled to aid in propellor selection?

Trimax drive system uses exhaust ventilation to reduce propellor loading and the available sales literature suggests acceptable performance at lower vessel planing speeds so this all looks good on paper..........

Has anyone else tried this approach?

[Northwester]

OK, I'm getting tired of waiting now.

81 views and not one reply?????

[RANCHI OTTO]

the top speed for 2500 kg and 1 x 210 bhp with surface piercing propeller will be in excess of 30 knots (up to 33 knots)

The geometry for a piercing propeller is very delicate,..try to take contact with ROLLA.

Install a light surface drive ...there a lot of makers like
Levi Drive,
Pulse Drive
Jolly Drive
Trimax
SDS
DBD
etc.

The ventilation on propeller is because the screw is well immersed behind the transom due to the l.c.g. position.
Ventilating the propellers using warm exhaust gas they work in mixed flow and the thrust is larger.

I have designed a lot of high speed boats with surface drives and I had never problems even at low speeds range.

Send an e-mail to SDS or ROLLA with the dimensions of the boat, engine data and they will calculate propeller dimensions, speed and l.c.g. opt.

OK ?

[Northwester]

Thank you Ranchi.

Your remarks are encouraging. It is not always safe or practical to travel at higher speeds in my waters because of danger of striking floating woody debris. That is the reason I am concerned with performance at low planing speeds of 10-14 knots with surface piercing design.