how can i run my boat?

i have made a diesel boat of metal that weights about 300 kg with engine without passenger.the engine is of 9 h.p and has 1.5 gear reduction and r.p.m of engine is 1500.the propeller pitch is of 7 inch pitch and 9.25 inch diameter.it submerges 1.5 foot below water.i want to
know will it run.i made these modifications this time.if it can run then how fast else.what modifications should i make.there are no marine engineers here.pls help

 

I'm not sure we have any marine engineers here at DIY, either. The questions you are asking are highly technical and I would be surprised if anyone can fully answer them, but we'll see.

 

Will it run? That depends on your engine and your boat. Does the engine run? Does the boat float?

How fast will it go? That depends on your hull design. You did not mention a gear reduction so I assume you are running 1:1. Theoretically the maximum is a bit less than 10 miles per hour. Take out propeller slip, thrust line and other inefficiencies and I think you could get 7 miles per hour with a clean hull.

Report back when you get the boat in the water. I would love to know if my guessing is correct.

 

I read the post several times and aside from the 4 rivers that run through Nepal, I am not sure where the OP would ever get it planed out on sand

 

I'm just hope the original poster comes back. I'm really curious to see a picture and to find out how fast it actually goes.

Oops, I see now that he has a 1.5:1 gear reducer. The theoretical top speed is about 6.5 miles per hour and with 30% of inefficiencies it will have a top speed of about 4.5 miles per hour. If the engine is in good condition I think you would do better if you could turn the gear reducer around so it increases the propeller's rpm. 7 inch pitch is pretty flat.

 

I think that it will run.actually i have flat hull and there is elevation at front.and the gear reduction is 1.5:1.this is my first boat and was really difficult for me becoz there were no marine engineers nor materials in our country.and i knew that other petrol engines had about 4500-5500 r.p.m and 2.08:1 reduction of yamaha.so i have even kept a gear box.actually i modified gear box in such a way that it runs faster in 1st gear than in second than 3 than 4th.and the reduction near prop is 1.5.so engine can turn that prop at higher r.p.m.for eg:after reduction also the prop of yamaha can achieve about 2500 r.p.m.and mine achieves 1000.and i have 2.5 times more torque for same prop(i have used same prop of yamaha 9.9 h.p 2 stroke)so i can increase r.p.m there to gain speed.any way i want it to run at speed of atleast 15 km per hour.thanx for help.and i am going to test tomorrow.

 

Actually i can get variable r.p.m from gear1000-1500-2250-3000.at what r.p.m should i run.i have used generator engine of "kirlosker".and that's all.

 

Oh by lack of time it is postponed up to friday.i want to know at what r.p.m should i run the prop that the force can be handled by engine.and i can get r.p.m from 1000-3000 while running by gear box.any more suggestions?

 

If you can change gears I would try them all and see which works best.

 

Such a nice of you.thanks.and one question to you:instead of directly keeping prop at suppose 2500 r.p.m is it better to run the prop at 1000 r.p.m and increase the gear and increasing the prop r.p.m as in cars?don't forget it submerger 1.5 ft below water surface.please.

 

It will depend on the propeller, your boat's hull and how much power the engine is actually producing. Generally small propellers are turned faster than big ones. I would start with the gear that turns the prop at 1'500 rpm with the engine at full rpm and see how that works. If the engine can not get to full rpm I would go with a lower gear to turn the propeller slower.

 

I understand that when r.p.m of engine is increased then the torque also increases.but the diesel engine produces more torque even at lower r.p.m.since i have flat hull and the prop submerges 1.5 ft below water can i run the prop at 2000 r.p.m when engine is at 1500 r.p.m?and when depth of prop increases under water high torque is required and can we go faster when prop's r.p.m is at 1000 and depth is 0.5 feet below or 1.5 foot below(engine r.p.m is 1500)?and isn't it that engines having 10 h.p power one has maximum r.p.m of 4500-5500(petrol)and another haing 1500(diesel) has different torque?as far i know that the petrol engines has 2.08 reduction(yamaha outboard) that means it's prop runs at about 2200-2700.so can't i also get that r.p.m by increasing gear though r.p.m of engine is 1500.and petrol engines has lower torque at lower r.p.m as compared to diesel so can't i run engine at lower r.p.m and prop at higher r.p.m.even my calculations shows that if my engine has 1500 max r.p.m and that yamaha has 4500-5500 them my engine produces 3.3 times more torque.

 

The depth of the propeller under water does not matter for a small boat as long as it is not drawing air from the surface. The torque to turn the propeller also does not change because you have it mounted deeper under water. If you had a submarine and were deep under water the high water pressure would help prevent cavitation but 1.5 feet is not enough pressure to matter much. Sucking air from the surface or from bubbles pushed under your flat hull will be a bigger problem.

Where did you get your propeller? What type of engine or boat was it from?

The best thing to do is just go out on the water and see what works best. Pay attention for vibration which is very bad. And, turning the propeller at higher RPM puts much more stress on the propeller. If it was ever damaged in the past or if you hit something under water the propeller may fail.

 

What problem may occur due to viarations?and i am also thinking to look for reasults on water rather to take more tension.and am i going to have problem at 1.5 ft below?the prop is of yamaha 9.9 h.p petrol engine 2-stroke.

 

That propeller normally runs up to 2'750 rpm so you should be good to turn it fast.

 

What problem occurs due to vibrations?

 

Vibration can destroy everything. If your boat is an inboard, propeller or shaft vibration can damage the shaft supports, stuffing box and transmission bearings. If it's real bad it can damage the hull. If the engine is not mounted properly the engine vibration can damage the motor mounts and break the engine loose.

 

Oh i see it.but u know that i have i have a base for everything.my engine lies inside the boat as like of engine fixed in chassis of car.and all the bearings are fixed on chassis.hope it will run tomorrow.only 17 hrs remaining.

 

I can not wait to hear how the boat runs. Can you post a picture?

 

Yeah sure.after 5 hr my boat will be on water.anyway i have no finishing on boat and it looks akward becoz this is my first boat.hope it will run.i am happy.

 

Oh no my boat didn't run actually the r.p.m of engine slowed very much.the engine is of 8 h.p and is that due to depth or what?there was some problem on engine before testing that is the valve was damaged and the piston too.i should have given that to mend.and my boat didn't run(r.p.m of engine decreased drastrictly) was that engine problem or the prop was dipped nearly 1.5 ft?shouldn't i have to run at 1:1 gear ratio?

 

Now i am going to make it in new way.that engine was damaged(it was a bad engine actually)now i am going to use 8 h.p engine of weight about 130 kg(diesel) and going to have 1:1 ratio with maximun engine r.p.m of 1500.i am going to use same prop of yamaha 9.9 h.p of 9.25 diameter and 7 inches pitch under 1.5 ft below water.will it work if the engine is on good condition?

 

Yes, it should work. Good luck.

 

Are you sure that 8 h.p(diesel) can run the 9.9 h.p(petrol) propeller at 1:1 ratio?under 1.5 ft below?any suggestions are welcomed warmly.

 

After doing some checking on the internet, it appears that the 9.9 Yamaha outboard had a maximum motor RPM of about 5500 RPM's. Through gear reduction, the prop would probably turn at half that speed or about 2700 RPM max. At 1500 RPM from your motor, the prop would be turning at 55 - 60% of it's maximum.

 

So can 8 h.p diesel engine run the same prop at 1500 r.p.m as engine has 1500 r.p.m without reduction under 1.5 ft below water surface?

 

***** Blog link removed***** that is my boat...well i forgot to tell about it to you....

 

So, the diesel did not work out? Nice new outboard though.

 

thnx pilot dane...sorry i forgot to contact you..i remembered after a long time