Beta 43 and PRM150 1:209 propeller size

[Tresfjord 26]

Hi.

I have a norwegian displacemeant / semi displacemeant Tresfjord 26 boat and im thinking about chancing the propeller.

8m long (waterline is 7.4m) and 2.8m wide, draft is 1m. Boat weights 2500kg fully loaded.

Beta engine is rated 43hp @2800rpm,  prm 150 gearbox is 1:2,09. It says on beta's website that the right propeller for beta 43 with gearbox ratio of 1:2 is a three blade 18x12. 

Would a 3 blade 18x13 propeller be over pitched? What kind of propellers do people have with this same engine and gearbox setup and what is the achiavable max engine rpm.

I dont want a propeller that is oversized and i wont be able to reach 2800rpm

My current propeller is a vetus 3 blade 16x14 and im getting maximum of 3000rpm and at cruising speed my propeller slip is 40%

 

Regards Henri

[Dalslandia]

Hi Henri

 

what max speed do you get today? at 3000 rpm

and what is hull draft without keel?

My guess so far is 9.5 kts and a 17-18X13

[Tresfjord 26]

19 minutes ago, Dalslandia said:

Hi Henri

 

what max speed do you get today? at 3000 rpm

and what is hull draft without keel?

My guess so far is 9.5 kts and a 17-18X13

If i remember correctly the top speed with the current propeller is only 8-8.2 knots. I cant go out to test it beacose the boat is out of water for winter.

Draft whitout keel is 50-60cm

I have been suggested a 17x14 propeller but i have room for 18 inch diameter propeller too.. and it would have a better bite to water

Cruising speed was 6.2knots @ 1900rpm.

Edited January 16, 2018 by Tresfjord 26

[Dalslandia]

You want some 17% of diameter from blade tip to bottom of hull 2,9" - 3", minimum 10% or 2"  or 50 mm.

18X12 - 17x14

My Winga 25 with Penta MD II have an 16X12 Think -11 would have been better

brother in law have an Albin 25 MD III 35 HP 2500, with an 17x14, (the Albin 25 is 1600 kg i Think) his is over reving some.

the Volvo gear is 1.9:1, that will be about the same prop RPM as you get at 2800/ 2.09:1

 

Edited January 16, 2018 by Dalslandia

[Tresfjord 26]

I have plenty of room for 18 inch propeller..

im just wondering what kind of propellers people have with this engine setup.  

Beta says that with 1:2 ger ratio (max shaft rpm 1400) the correct propeller is 18x12

2800 engine rpm / 12 pitch = 233 engine rpm per 1 pich

But with my setup 1:2.09 max shaft rpm is 1339.

1400rpm - 1339rpm = 61rpm shaft rpm

So 61 shaft rpm x 2.09 gear ratio = 127 engine rpm..

It seems if i want a 18 inch diameter propeller it should be 18x12.5... i dont know if any company sells those.

Edited January 17, 2018 by Tresfjord 26

[Dalslandia]

I saw Pictures of Tresfjord 26 boat hull on the net, so can just agree that you have room.

 

I used the Propking spread,

the pitch is just not completely linear if you change the diameter, with smaller diameter the Power put more speed to the water, like a jet, and the otherway around if the diameter is greater.

so a 18" diameter will have a less pitch then a 16" all other things stay the same.

speed/ RPM = pitch, problem is Wake and slip

Diameter is depending on Power and RPM

Pitch is depending on speed, rpm Wake and slip

Wake fraktion depends on hull shape and draft

I come Close to 18x12

but was thinking that you said you got 3000 rpm and 8,2 kts, do you Think the Engine is rev limited? if so the speed with a better propeller will be a lot higher

The prop king predict 9.5 kts with 85% Power at the propeller, and you are Close to 18x13.

if you get it wrong, a prop shop could shange the pitch an inch or two

 

 

Edited January 17, 2018 by Dalslandia

[Bee]

As you say Beta suggest 18 x 12. I have the same engine and gearbox and I used to have a 17 x ??   and the engine did not feel as though it was working at all hard, I changed it for an 18 x 12, the engine now feels and sounds as though it is working harder and the whole thing feels about right, no idea if it will rev to 3000 but 2000 is plenty quick enough for rivers and 1400 for canals.

[Dalslandia]

Henri, most people online here have steel narrow Canal boats of 10-15 tons and hardly ever go faster then 4 MPH (3.5 kts) unless on a river, and the inflow to the prop is limited by hull and shallow waters

[Tresfjord 26]

15 hours ago, Dalslandia said:

I saw Pictures of Tresfjord 26 boat hull on the net, so can just agree that you have room.

 

I used the Propking spread,

the pitch is just not completely linear if you change the diameter, with smaller diameter the Power put more speed to the water, like a jet, and the otherway around if the diameter is greater.

so a 18" diameter will have a less pitch then a 16" all other things stay the same.

speed/ RPM = pitch, problem is Wake and slip

Diameter is depending on Power and RPM

Pitch is depending on speed, rpm Wake and slip

Wake fraktion depends on hull shape and draft

I come Close to 18x12

but was thinking that you said you got 3000 rpm and 8,2 kts, do you Think the Engine is rev limited? if so the speed with a better propeller will be a lot higher

The prop king predict 9.5 kts with 85% Power at the propeller, and you are Close to 18x13.

if you get it wrong, a prop shop could shange the pitch an inch or two

 

 

The of gear engine rev is limited to 3000rpm. I dont know how much above 3000rpm it would rev.

I think im going to go with the 17x14, 3-blade vetus P3B type propeller (current one is vetus P3B allso)

And if im having trouble with engine over reving its better to add pitch to a larger propeller

With my and propeller specialists calculators a 17x14 should be just right for my boat.

[Dalslandia]

Without CFD and tow tank test, all that remain is to do practical test.

I understand the hull is half planing type?

with 85% of the nominal Power I get 9.5 kts and a 17.5 x 12.7 average of two ways to calculate.

at 9 kts 17.6 x 12.2

at 8.5 kts 17.6 x 11.7

when limit the diameter to 17.0" the pitch would go up 1" - 1.25" making it a 17x14

I Think you are correct with this.