Engine Sizes

[Guest]

I am repowering my 43T "Ulu" and thought I would jot down the calculations I've used to work out the horsepower for the new engine. I dont pretend to any engineering knowledge and the following is based on "Marine Diesel Engines" by Nigel Calder. In the end the Yanmar 4JH4-TE turned out to be about right. Details of the calculation, based on Ulu, are as follows. I've done the same calculation, further below, for the 38MKII and the 33.

For the 43T:

1. Speed length ratio -
(generally adopted max for displacement hull
is 1.34). = 1.34
2. Length at the waterline - LWL in feet expressed as decimal = 36.83 feet

3. SL ratio multiplied by the square root of the LWL gives Hull Speed = 8.13 Knots

4. Resistance per long ton in lbs
(This is 55lbs for a sl ratio of 1.34) = 55 lb

5. Displacement = 37000 lb

6. Resistance for the boat = 908 lb

7. Effective Horse Power = resistance X speed X .003 = 22.2 hp

8. Propeller efficiency rating, say about the average = 40%

9. Horsepower required to drive boat at hull speed = 55.41 hp

10. Add the standard 33% for adverse
conditions = "max" HP required in practice = 74 hp

For the 38MKII:

1. Speed length ratio = 1.34

2. Length at the waterline = 33.0 feet

3. SL ratio multiplied by the square root of the LWL gives Hull Speed = 7.70 Knots

4. Resistance per long ton in lbs = 55 lb

5. Displacement in pounds (27,500 plus gear) say = 29,000 lb

6. Resistance in pounds for the boat = 712 lb

7. Effective Horse Power = resistance X speed X .003 = 16.4 hp

8. Propeller efficiency rating = 40%

9. Horsepower required to drive boat at hull speed = 41.11 hp

10. Add the standard 33% for adverse
conditions = "max" HP required in practice = 55 hp

For the HC33 the same calcs give 34 hp.

The above calculations give a pounds per horsepower of 502 for the 43T and 530 for the 38MII. Apparently 1 hp per 500 lb is considered the max power which should be needed.

The other thing I learned from this is that there are quite big gains to be made in prop efficiency. With a prop at the top of the range of efficiency, the 43T would apparently be pushed to hull speed by a 59 hp engine.

[rob_leacock]

I am repowering my 43T "Ulu" and thought I would jot down the calculations I've used to work out the horsepower for the new engine. I dont pretend to any engineering knowledge and the following is based on "Marine Diesel Engines" by Nigel Calder. In the end the Yanmar 4JH4-TE turned out to be about right. Details of the calculation, based on Ulu, are as follows. I've done the same calculation, further below, for the 38MKII and the 33.

For the 43T:

1. Speed length ratio -
(generally adopted max for displacement hull
is 1.34). = 1.34
2. Length at the waterline - LWL in feet expressed as decimal = 36.83 feet

3. SL ratio multiplied by the square root of the LWL gives Hull Speed = 8.13 Knots

4. Resistance per long ton in lbs
(This is 55lbs for a sl ratio of 1.34) = 55 lb

5. Displacement = 37000 lb

6. Resistance for the boat = 908 lb

7. Effective Horse Power = resistance X speed X .003 = 22.2 hp

8. Propeller efficiency rating, say about the average = 40%

9. Horsepower required to drive boat at hull speed = 55.41 hp

10. Add the standard 33% for adverse
conditions = "max" HP required in practice = 74 hp

For the 38MKII:

1. Speed length ratio = 1.34

2. Length at the waterline = 33.0 feet

3. SL ratio multiplied by the square root of the LWL gives Hull Speed = 7.70 Knots

4. Resistance per long ton in lbs = 55 lb

5. Displacement in pounds (27,500 plus gear) say = 29,000 lb

6. Resistance in pounds for the boat = 712 lb

7. Effective Horse Power = resistance X speed X .003 = 16.4 hp

8. Propeller efficiency rating = 40%

9. Horsepower required to drive boat at hull speed = 41.11 hp

10. Add the standard 33% for adverse
conditions = "max" HP required in practice = 55 hp

For the HC33 the same calcs give 34 hp.

The above calculations give a pounds per horsepower of 502 for the 43T and 530 for the 38MII. Apparently 1 hp per 500 lb is considered the max power which should be needed.

The other thing I learned from this is that there are quite big gains to be made in prop efficiency. With a prop at the top of the range of efficiency, the 43T would apparently be pushed to hull speed by a 59 hp engine.

[warmrain]

Rob,

I think I know your boat, from birth in fact... If it is the boat I'm thinking of her original name was Non-recourse.

If you'd like to talk about it write back or PM or email...

[rob_leacock]

Steve,

Thanks for your note - the pm function seems to have been disabled. Yep, I've been told that was her original name. She has a distinctive teak doghouse. I bought her from Mike and Christine Wiggins, (who posted an article on this website about installing a Capehorn windvane on her). Would love to hear anything you can tell me about her. If you prefer my email address is rob@leacock.com.au

She's a great boat. I've decided on a Yanmar 4HJ4-TE for the repower. One unusual feature is the automotive type drive shaft between the tranny and the prop shaft. The two shafts are connected by a unversal joint. Do you know if this is common? I'm wondering how much trouble I should go to replace this setup with a straight through prop shaft from prop to transmission.