Prop calculations

Looking for advise

I am currently building a professional fishing boat based on a planing sports fishing hull of 8m50 length 3m40 width
with variable deadrise. I have lengthened the hull by 1metre and repositioned the engine about 2,5metres in from the transom. The engine is a Perkins M215 (215hp) which runs at 2500rpm max and drives a shaft into a shallow tunnel. The boat weighs theoretically 4tons empty and 6ton with fuel, fishing nets, various equipment
and captured fish (hopefully).
I have asked several prop and rudder suppliers to calculate reduction ratio, prop size and pitch and rudder size and shape. I would hope for 20knots flat out and 15knots cruising speed.

They suggest Ratio Prop size, pitch, type
1 1.56 24"x85% 4 blade (based on 6tons)
2 1.22 18"x16" 4 blade (based on 5tons)

They are also suprised by the empty/loaded weight differential

The hull design (31') built as a fast flybridge fisherman (5tons) was equiped with 2x230hp yanmars and gave 30knots top speed.
Geoff
Uniquest Marine

 

Geoff, before I answer your question, where are you planning to fish from?
Which location?

 

Looks Like those numbers are in the ballpark, How much diameter can fit in the tunnels? I suspect also your cruise will be either faster or slower as 15 kts. will probable not be a efficient speed, just a hunch-.

I came up with 18"x18" 4 blades with 1.22 reduction and 6 tons
or 22x24 4 blades with 1.5 reduction and 6 tons If you are going to spend most of the time at non planning speeds I would go with the biggest diameter that will fit with good clearence and a higher reduction ratio.

Good luck

 

Our fishing area is arround cap d'Agde on the south west Med coast
If you are thinking of the sea state arround here then it is very choppy, ie short sharp though not very deep troughs.
Geoff

 

My customer wants to be able to get to his fishing spots as quick as possible, once there he would explore the area at 7-8 knots (not on the plane) so I think I should give preference to the faster 1.22 ratio. Given that the motor has 470lbs/ft torque at 2000rpm and 510lbs/ft at 1400rpm, I imagine that the motor would not be overly stressed.
I don't want to chose an extreme configuration, what would be the efficient speed with this config.
Geoff

 

Cap d'Agde

Hello Geoff,

Yes I know where it is - if I am in France I often berth at Port Camargue or
Aigues Mortes.....

 

Paul Bezzi, owner / CEO of Renou Dardèl - France Hélices has his Med. office in Cannes-La Bocca. That's about 250 - 300 kms; I'll ask if they have a subsidiary in Marseille; if you need a prop I will make sure they give you a good rate!

 

I've just received a quote from France Helice (M. Launay) based on a 1.22 ratio,
but he doesn't seem to happy about the ratio, he prefers 1.56.

I've tried PROPCALC.XLS which seems very good and can obtain more or less 20knots
with both 1.22 and 1.56 with prop dia.s of arround 19" and 22" respectively. The problem is the Perkins turns slowly and has a lot of torque, I don't want to overload the engine but I would like to have fast boat, the hull is a planing hull. I have plenty of room in the shallow tunnel for a large prop.
Can you advise me?

Geoff
Uniquest Marine

 

I am tomorrow in the office, there I keep my design stuff, but I do better and give Paul (Bezzi) a call and you will have the best service - they are the foremost best in France anyway. Leave it to me - mail me your telno
Regards

Oh yeah, I will be somewhere in june in the Med with my new boat, I'll come than straight from LA - will definately drop down in Port Leucate.
Can I come in there? Draft little over 10'......

Brien

 

Hi Brien,
Thanks in advance for your offer concerning France Helice.
I had M. Launay on the phone this morning, he is going to redo his calculations with a 1.56 ratio. I'll let you know the outcome.
Port Leucate has a max depth of 3metres.
Geoff

 

Are there any freeware prop calculators on line?

I only need to figure a replacement prop for my boat that has the unfortunatly wrong transmission in it.

Previous owners removed a 1.5 to 1 and installed a 3 to 1 , so even with a bigger prop the performance suffers.

The great Dania FL nautical flea market is coming up March 31 , and I would love to know that to hunt for.

FAST FRED

 

Thanks for the file it looks like it takes a lot more into account than the propcalc.xls
Questions: how is SPH defined, is it shaft horse power?
Do I read the speed graph at the 209hp level which gives approx 15knots, why does the graph show more hp than I specified? Does this mean it is impossible to get 20knots
Geoff
Uniquest Marine

 

Geoff, Check out the spread sheet again, and make sure the bold inputs are correct. Shp is Shaft horse power and is based on "Average loss" increase the number of bearings and loss will increase. All this assumes that LCG is optimum, and that bottom loading is not too much to reach the desired speed, and does not take wetted surface,or surface roughness into account.