Project 31 Prop Size

[jimthesail]

Ahoy !
Have a pair of 15 x 11" props on a pair of Volvo Penta 270 legs connected to Ford/Indenor/Peugeot XD2 diesels which give 5hp more than the original MD21a's. Pretty sure these are wrong and need to be nearer 15" pitch.
Can anyone confirm the original fitted please and if anyone knows
where to get ?
Jim

[DougR]

If you can supply some data, I'll run it thru the prop program....and we will come pretty close.

Boat data:
LOA
LWL
BEAM
WEIGHT FULLY LOADED

Engine data:
Max H.P.
Max RPM
Sterndrive reduction ratio

Back when the MD21 was in production there were no prop programs, just curves to plot length and weight against...then you showed up at the boat builder with an arm full of props and started testing. You will source the props for the 270 drive at a VP dealer.

DougR

[jimthesail]

Hi !
Project 31
o/a 31ft
w/l 28
Beam 10ft
Displace 4 tons
70hp x 2
rpm 4500
1.9/1
Draught 1 1/2 ft ?

Understand that it planes up to approx. 16k e.g. Princess Yachts rebuilt Project 31 with twin MD21a's YouTube
(These are 65 bhp, my XD2's are 70 bhp)
Thanks for help.
Jim

[DougR]

Are you certain about the drive leg reduction ratio? Common ratios were:

1.61:1 - usually fitted on V-8 engines 200+ h.p.
1.89:1 - fitted on inline and V-6 engines about 170-175 h.p.
2.15:1 - fitted on four cylinder engines around 120-140 h.p.

The original sales brochures that I have for the AQD 21 with the 280 drive show the 2:15:1 ratio was used.

You find the drive ratio stamped into the ID tag riveted on the top of the drive just behind the u-joint bellows, or stamped into the face of the prop shaft bearing housing behind where the zinc mounts.

The boat will perform and accelerate best with the 2.15 ratio, but you have what you have.....

Double check and let me know.

DougR

[DougR]

I ran the numbers with the info you provided. I used an inboard program because the engine/sterndrive package you have is no longer in the sterndrive portion of the prop program, but the differences will be minimal. The sterndrive will usually perform slightly better than an inboard, but it's a good place to start.

Ratio 1.89:1 Max speed- 16.2 Kts. Optimum 15 " prop - 15x11x 3
Thrust- 1590 lbs. Efficiency- 58%

Ratio 2.15:1. Max speed- 16.2 Kts. Optimum 15" prop - 15 X 13 X 3
Thrust- 1630 lbs. Efficiency - 60%

Volvo Penta Props for the 270 drive:
15x 11 X 3. L.H. part # 813296. R.H. part # 813297
15 X 13 X 3. L.H. Part # 813316. R.H. Part # 813317

You also might be able to find other brands of propellers which will fit the 270 drives....don't know what is available in your area.

I suggest starting with the props you have and see what speed and RPM you achieve. You should get close to 4500 RPM at full throttle with a clean hull and moderate to heavy load.

DougR

[jimthesail]

Hi !
Thanks for info.
Having 'twiddled' the input shaft I find it rotates under two revolutions for one rev. of the prop so you are right they are 2.15 to 1's ! From your figures it would appear that it has had the props from a 1.9 fitted at some time ?
Cheers !
Jim

[DougR]

If the input shaft turns less than two turns for one turn of the prop shaft, then the ratio is probably 1.89:1. If the input shaft turns more than two turns for each one turn of the prop shaft, then y have a 2.15:1 reduction.

Give it a try with the existing props, then you will know if changes are necessary.

DougR

[jimthesail]

Hi !
I got my 'props in a twist' over this ! My original statement was wrong !
If the input travels two revolutions to get less than one rev. on the prop. so the ratio is 2.15 ? Ie. it would take more than two revs to get one rev. out of the prop. therefore it would likely be 2.15 ratio ? Therefore the ratio is lower than 2:1 ?
Jim

[coopec43]

Originally Posted by jimthesail

Ahoy !
Have a pair of 15 x 11" props on a pair of Volvo Penta 270 legs connected to Ford/Indenor/Peugeot XD2 diesels which give 5hp more than the original MD21a's. Pretty sure these are wrong and need to be nearer 15" pitch.
Can anyone confirm the original fitted please and if anyone knows
where to get ?
Jim

Does this help?

https://www.vicprop.com/calculator.htm

Clive