What propeller to fit 40 mm shaft to reach minimum speed

Soulayman

Soulayman

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Hi everyone, I changed the engine to Fiat 130hp and the gearbox to twin disc 2:1 ratio of my wooden 11m boat. I keep the the 40mm shaft and the propeller of three blades. I have problem with the speed. I need to reach minimum speed when working to avoid using the gearbox Stop and forward. If I reduce the size of the propeller do I make my boat drive at less speed ? Or do I need to change a propeller with 4 blades and less pitch for smooth and less vibration drive? Did the propeller with 4 blades make the boat drive slower in minimum speed ? Thanks
 
First shaft size is irrelevant for prop sizing, although 40mm is fine for that HP and reduction. You don't say what you changed from so it is impossible to guess what you need to do with the prop you have. Much better to start from scratch and do the calculations for what you now have - surprised you did not do this when you invested in a different engine. As first cut suggest you enter the data for your boat/engine/reduction into this programme vicprop.com/displacement_size_new.php This will give you a ball park size to meet your required cruising and maximum speed (assuming the HP is approptiate for the displacement and hull form). However as you will almost certainly need a new propeller, better to go to a specialist such as Clements Engineering, Lake Engineering , Hamble Propellers or one of the many other prop specialists Google will give you. They will do the calculations for you and recommend a size. They are then responsible if they get it wrong - although they are experts and mostly get it right first time.
 
Thanks for the link you provided, it's helpful. I changed the old marin engine 48hp buck with original Gearbox 2:1 ratio to Fiat industrial engine 8062.02 . 130hp connected to twin disc 2:1 gearbox . Know that my actual propeller with 3 blades is about 20" diameter. Many thanks
 
Soulayman said:
Hi everyone, I changed the engine to Fiat 130hp and the gearbox to twin disc 2:1 ratio of my wooden 11m boat. I keep the the 40mm shaft and the propeller of three blades. I have problem with the speed. I need to reach minimum speed when working to avoid using the gearbox Stop and forward. If I reduce the size of the propeller do I make my boat drive at less speed ? Or do I need to change a propeller with 4 blades and less pitch for smooth and less vibration drive? Did the propeller with 4 blades make the boat drive slower in minimum speed ? Thanks
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Not enough info to make a sensible comment : hull material and shaft diametre are irrelevant (see Tranona).

You need: Hull type (displacement/planing), displacement of boat, DWL, prop/keel configuration and prop clearances, prop diametre, pitch, no. of blades, blade area, engine RPM and power curve, desired speed and use of vessel (yacht, work boat, towing, fishing, if yes, what type etc.) .

Four bladed props are only of interest if:
a) not enough space for a prop of required diametre .
b) blade loading too high.
 
Speak to the good folks at Darglow, and they'll give you qualified information and direction. They're generally a 'yottie' prop maker, but are a good starting point.
 
Soulayman said:
Thanks for the link you provided, it's helpful. I changed the old marin engine 48hp buck with original Gearbox 2:1 ratio to Fiat industrial engine 8062.02 . 130hp connected to twin disc 2:1 gearbox . Know that my actual propeller with 3 blades is about 20" diameter. Many thanks
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That seems a really odd thing to do - fitting an engine with nearly 3 times the hp. As I understand your initial post you are having trouble with going too fast at low engine speeds - not surprising with that amount of hp. I can understand 130hp on a heavy semi displacement 11m motor boat which is what I assumed you have, but if the boat previously worked with 48hp then maybe you have the wrong engine.

More details of your boat as suggested in post#4 would be helpful.
 
Tranona said:
That seems a really odd thing to do - fitting an engine with nearly 3 times the hp. As I understand your initial post you are having trouble with going too fast at low engine speeds - not surprising with that amount of hp. I can understand 130hp on a heavy semi displacement 11m motor boat which is what I assumed you have, but if the boat previously worked with 48hp then maybe you have the wrong engine.

More details of your boat as suggested in post#4 would be helpful.
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Thank you. I don't think it's engine problem , many have same boat like mine they put 160hp engine inside . If I change the gearbox to 3:1 ratio than I will have 1/3 RPM of the engine , and that will result less speed. I'm thinking to change the propeller, a bigger propeller means less prop RPM than the actual one. Many thanks
 
Lack of information, as noted above. Bigger props at lower speeds work better. But, lots depends on your hull and the expected speed?
Since others have put bigger engines in similar hulls, is there no information available?
Bigger propellors put more load on the engine, but that also depends on the pitch and the RPM/reduction ratio..
 
Soulayman said:
Thank you. I don't think it's engine problem , many have same boat like mine they put 160hp engine inside . If I change the gearbox to 3:1 ratio than I will have 1/3 RPM of the engine , and that will result less speed. I'm thinking to change the propeller, a bigger propeller means less prop RPM than the actual one. Many thanks
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No. A slower shaft speed will need a substantially bigger diameter propeller.

You really need to go back to basics and put the data on your boat into the programme I suggested. This will give you a good idea what speeds you can expect with the engine and reduction ratio and the optimum propeller. Propellers are sized so that you can achieve close to rated HP and the speed required - assuming this is a semi displacement or very heavy displacement boat. Once you get the correct propeller (which may need a change in reduction ratio) the slow speed running takes care of itself.

Still intrigued why a boat that had a 48hp engine jumps to 130hp. Clear that a prop suitable for the original is not going to work with nearly 3 times the HP, hence the need to do the calculations to arrive at the best combination of engine/gearbox/propeller for the boat.
 
DownWest said:
Lack of information, as noted above. Bigger props at lower speeds work better. But, lots depends on your hull and the expected speed?
Since others have put bigger engines in similar hulls, is there no information available?
Bigger propellors put more load on the engine, but that also depends on the pitch and the RPM/reduction ratio..
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Thanks for the reply, others have similar engine placed in bigger propeller and bigger shaft,(60mm) they also have a different gearbox ratio which is 3:1. And about the hull of my boat it's a Deep V hull with 4 feet and 33 feet long. I will try this, I'm going to change the propeller with a bigger 3 blades a little bigger diameter and keep the shaft 40mm. My biggest worries that the shaft will bend. The point that I want to reach is to drive the boat to minimum speed when working. Let say at 1knt /h . I'm not looking for the super speed with full RPM when traveling , I'm happy with 7knt /h . I changed this engine for reason of more speed , to travel fast but now I'm facing a different issue , the lowest speed still too fast for working, imagine to put forward for 10 second and stop for a min and again put forward for 10 second , doing this every fishing night for 8hours straight may broke my gearbox twin disc in the middle of the sea. How big the propeller a 40mm shaft with 5feet long can Handel in half speed 1000RPM. Many thanks
 
I'm sorry to say, but you are going at this backwards. Prop diametre depends on engine hp, engine RPM and gear reduction i.e. shaft speed and in that order. Shaft diametre depends on prop diametre; here rule of thumb prop diametre divided by 17.5 for a monel shaft, 14 for bronze. Next is max distance of shaft between bearings which for a 40mm shaft works out to be about 5.75'.

A larger diametre prop will, providing the engine has enough power and tip speed on the prop does not get too high as to cause cavitation, not make your boat any slower. If speed at tick-over is to high to do your work, you may want to consider towing some type of drag , such as a couple of buckets or a sea anchor.
 
Soulayman said:
Thanks for the reply, others have similar engine placed in bigger propeller and bigger shaft,(60mm) they also have a different gearbox ratio which is 3:1. And about the hull of my boat it's a Deep V hull with 4 feet and 33 feet long. I will try this, I'm going to change the propeller with a bigger 3 blades a little bigger diameter and keep the shaft 40mm. My biggest worries that the shaft will bend. The point that I want to reach is to drive the boat to minimum speed when working. Let say at 1knt /h . I'm not looking for the super speed with full RPM when traveling , I'm happy with 7knt /h . I changed this engine for reason of more speed , to travel fast but now I'm facing a different issue , the lowest speed still too fast for working, imagine to put forward for 10 second and stop for a min and again put forward for 10 second , doing this every fishing night for 8hours straight may broke my gearbox twin disc in the middle of the sea. How big the propeller a 40mm shaft with 5feet long can Handel in half speed 1000RPM. Many thanks
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There seems to be a serious mismatch of expectations here. First a deep V hull is inappropriate (and inefficient) for your purpose (as you have explained it). If you want low speed handling and a top speed of 7 knots or so then you just need a displacement hull which at 11m would displace around 5-6 tonnes and require around 45hp to achieve 7.5 knots (theoretical hull speed), although typically on a working boat with variable loads and operating in adverse conditions 60hp would be a good choice to give you potentially up to 9 knot.

As it is now you have far more power than you can ever use but you don't have enough power to get on the plane and as you have discovered get all the downsides of excess power in poor low speed handling.
 
Apart from Tranona's good advice. It seem the only way you can get to your 1kt working speed is to reduce the engine speed to less than 1,000rpm or fit a prop with finer pitch. This does mean that your new big engine will be working under little load, which is not very good for diesels.
 
Laminar Flow said:
I'm sorry to say, but you are going at this backwards. Prop diametre depends on engine hp, engine RPM and gear reduction i.e. shaft speed and in that order. Shaft diametre depends on prop diametre; here rule of thumb prop diametre divided by 17.5 for a monel shaft, 14 for bronze. Next is max distance of shaft between bearings which for a 40mm shaft works out to be about 5.75'.

A larger diametre prop will, providing the engine has enough power and tip speed on the prop does not get too high as to cause cavitation, not make your boat any slower. If speed at tick-over is to high to do your work, you may want to consider towing some type of drag , such as a couple of buckets or a sea anchor.
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Thank you Laminar , dragging something behind the boat is the easiest solution, many thanks for the prop information.
 
Soulayman said:
Thank you Laminar , dragging something behind the boat is the easiest solution, many thanks for the prop information.
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Noi its not!

Seems to me you need professional advice from a qualified engineer, who has actually seen your boat and engine. Few boat owners have the skill to calculate prop sizes, so you are in good company.

Prop size calculation is well know to be a dark art, and althogh the advice given above is sound, unless you know how to set about it, you will be wasting time and money tryuing to guess the answer to a long a fairly complex mathematical formula involving dispalcement, hull shape, hull speed, engine revs, gearbox ratio, speed through water, prop diameter and prop pitch, and a good few years engineering experience in the field! . Anyone one of those adrift and you will get the wrong answer and a prop that still doesnt do the job properly!
 
Just as a heads up, dragging something behind will slow you down 'til you put it into reverse, when it becomes FUBAR.
 
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Thanks to all for your advices, @jamie N, dragging something behind the boat will do the job. And to avoid the reverse problem, that thing should fleet to stay away from the prop. The idea is still blurry , but a small ball supporting the thing that will be dragged behind the boat will dive in farward and fleet when I stop or In reverse. Many thanks
 
@oldharry ,you are definitely correct. The biggest mistake is to buy an engine without considering the minimum speed, all my concern was the maximum speed. @jamie, I'm thinking to dray something behind the boat that will fleet in Stop or Reverse.
 
Hi everyone, I removed the cable connected to the gearbox twin disk mg 506 and I tried to turn the valve gently and slowly till I make the shaft start turning slow than it it runs to normal speed. I didn't hear any harsh engagement or crunch in the Gearbox , too many told me that the hydraulic gearbox make that sound in the engagement, if that's the case so why the engagement is much smoother manually ? Many thanks
 
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