Shaft Angle

I'm pretty new to boat building and boats in general for that matter. I was wondering if someone could explain the advantages/disadvantages of having a 9 degree angle on the drive shaft vs a 12 degree angle. I'm building the drive system from scratch so I dont need to match the angles to a v-drive or any pre-existing drive setup. The boat is a 16' Rayson Craft (bare hull) Like I said I am new to the boat thing, so if I need to include any other info that would help let me know.

 

The ideal shaft alignment is parallel to the LWL. Other wise as close to this as practical, so a 9 degree shaft is placing more thrust in the direction you want to move, then a 12 degree, so it's better.

 

Also when reversing, having caught that huge fish, will NOT pull the stern down, and drown the boat if you are not looking at the transom...

 

Logically thats what I was thinking, but I'm by no means an expert when it comes to boats. I wasn't sure if there was some other reason for having the prop angled down more. -Thanks for the info.

 

Only reason I can figure is so that the propeller, when rotating does not cut big holes in the bottom of the boat

 

Bigger shaft angle is not necessarily bad if it results in a better running attitude, as in not having the bow pointing to the sky.

 

That's right. You also have to have sufficient tip clearance between the top of the propeller and the bottom of the boat, and that should be 15% to 20% of the diameter of the propeller. If the propeller is too close to the hull, it can suffer from cavitation and erosion. Vibration in the hull from the propeller will also be pronounced.

Eric

 

Using your own optimal angle means you also have to make your own or modify a standard shaft support.
Bear in mind that the distance between the center of the output shaft (gearbox or engine) and the bottom of the boat demands a certain prop shaft angle. Moving the engine forward reduces the angle, but in a 16 ft hull you do not have much choice.

 

Thanks for the info about how far the prop needs to be away from the hull. That was one other thing I was curious about, so that took the guess work out of it! One other question you could maybe help me with??? How far forward should the prop be from the transom? Would it be a bad thing if it was basically flush with the transom? I'm going to be using a transom mounted rudder assembly, so the rudder wont be in the way of the prop being back that far.

I didn't want to make my own, so I will be using equipment that is readily available. I was thinking 9 degrees or maybe 12. As far as the motor is concerned, my dad and I have always wanted to do a hybrid setup (electric propulsion, gas range extender) The motor I will be using is a 9 inch NetGain motor ( http://www.evsource.com/tls_warp9.php ) running in the 144-200 Kilowatt range as far as power output is concerned. That's about 190-270 HP if my math is close, but that's a whole different topic... If I use a 9 degree strut and other gear, that puts the motor right between the front seats, 7ft forward from the transom (typically where the v-drive box would be) The battery pack will be back against the transom, so I'm hoping that will equal out the weight distribution from having the 145lbs motor that far forward on the boat.

 

Oh god the hybrid thing again.

NS,

Abandon the hybrid idea now. Or do a quick calculation on how much weight in batteries you will need to run that engine for any period of time. If you are using a 144kw engine, then for every 6 minutes of run time at full power you will need 14kw of electrical power. A large deep cycle battery can hold a nominal 12kw at 12v with a weight of around 100lbs. However you can only really draw down 50% of a battery before you cook it, so you need something like 200lbs of batteries for every 6 minutes of operation.

Your generator isn't a range extender, it becomes the controlling factor in you range, because boats cant carry enough batteries to add appreciably to range.


Now if you said you wanted to use a very efficient hull, operate in the 10kw range, and had a big boat, and filled it completely with batteries you might be able to use a generator for range extension.

 

Well Stumble, you raise some good points, but a few things I'll have to disagree with. Much like the car I drive, It may have a peak power output in the 120-130 hp range. Do I drive constantly at that powe output? Well no... I know that the batteries wont last that long at full power output. And I really don't need them to last that long at full power output. I dont plan on going long distances, just go out make a few speed runs, cruise around a little, and pull the fishin pole out for awhile and relax
You obviously know much more than I do about boat design, and since I appreciate 'constructive criticism' maybe you could help me figure a few numbers father than just say 'Chang your design!' How much power do you honestly think it'll take to make that boat plane? and at what weight? A Mercury 5.7 weighs in at 860lbs fully dressed according to Mercury's specs. I'll be running half that weight in running gear and batteries (lithium 72volt 100Ah 160lbs/motor 150lbs)
Like I said, I came here for help and constructive criticism. And maybe if I only need the boat to run all out for six minutes and then I'm done...this setup is just right You never asked what I was using the boat for... So anyways, if you could help with any info on the power/weight/drag issues of how much power I need, (since you brought it up) it would be greatly appreciated.

 

You need to look at Glen-L boats and see how their boats are set up . Better to have the rudder under the hull not stuck on the back !! Thats not classy at all !!
The prop shaft needs to be as low a angle as possible . If the nose comes up fit a cav plate right across the back of the hull .
Time you went and look at how some of the old flaties boats from yester year were set up and there reasoning !!!.Remember you are not re-inventing the wheel ! its all be done before many times , seek some advice from some one that been there done that and save a heap of time and money . Scream and fly guys would be a good place to go for a ask and look see maybe !!

 

Thanks tunnels, I'll check it out. I dont want to reinvent the boat part, thats why I'm here.

 

GlenL had a boat called Missile that was pretty close to the one you have !!

 

...also remember that the gearbox can have a built in inclination instead of straight, often 9 or 10 degrees, so you can keep your engine parallel and offset the box and shaft.