You heard it here first: Low Profile "Hidden" outboards coming soon.

well
there are plenty of race boxes that are 1:1 and or slightly over/under drive.
Thats makes a very small gearbox but with very low torque so the diameters are small not good for much except top speed

Reading the above posts, sounds like you can only have a efficient prop if it is going slow as a large diameter to make up for the short cord length will create so much drag it wont go anywhere fast.
I wonder when the drag overcomes the higher efficiency?

 

anywhere you gain gen set power and only have one engine on board and still have propulsion is a step forward.
lets say 40' yacht that needs main engine plus needs a gen set, what a waste as usually only one running at a time

 

Yes, that was the supposition and I don't know which method gives the smaller diameter gearbox or if anyone has tried to find out . Just guessing at it did not seem productive.

 

Just to clarify the prop efficiency thing a bit more.

All the prop is doing is accelerating a mass of water backwards to provide thrust (by reaction) to propel the boat forwards.

If you run a few numbers you soon find that best efficiency is given by accelerating a bigger mass of water to a speed that's only just faster than the boat speed to get the needed thrust.

Because thrust is given by the product of mass and velocity squared, you can get the same thrust by accelerating a small mass of water to a higher velocity, which is what a small, fast turning, prop, or a jet drive, does. The snag is, because the prop is spinning faster, it's own blade drag losses are greater (blade drag being proportional to the square of blade speed), plus the viscous losses from the faster outflow velocity are greater.

Ideally you want a prop that at maximum cruise speed is delivering most of the thrust from the mass part of the thrust equation, rather than the velocity squared term.

 

thanks for that Jeremy
Would you have any graphs showing a boat from idle to say 30kts diameter wise
Lets say its an outboard so 200hp at 5000 which means 200ftlbs at 5000 as well ( close)
and it will go to say 30kts?
How much diameter difference are we talking from best to worst, boat weights 1500kg and planes?
choose any ratio you like
thanks for the education
Regards

 

Sorry, but it's a bit of an iterative process to match the prop efficiency to the hull resistance over a wide range of boat speeds, so there's no one, simple, curve that will tell you the optimum diameter, rpm, blade chord, number of blades or blade pitch, I'm afraid.

The trade offs are pretty complex, because as boat speed increases the prop diameter has to be reduced in order to keep the blade tip speed (which is a function of prop rpm, prop diameter and boat speed) within bounds. The tip problem is complex, but can be simplified a bit by only looking at minimising tip vortex shedding losses, which reduce efficiency (these probably account for a large part of total prop losses at high speeds). Relative inflow velocity across the prop disc also plays a big part, as the relative velocity at the bottom of the prop disc can often be significantly higher than that at the top of the prop under some conditions, due to the combined effect of the hull boundary layer and the disturbed flow from the leg on an outboard.

The result is that the relationship between diameter, rpm, blade chord, pitch and boat speed becomes too complex to accurately model.

The complexity of designing and selecting a prop for a planing boat has led to the plethora of different empirical methods that are around, together with the general view that prop selection is as much about art and experience as science. Often it's a process of trial and error, complicated by the fact that the most efficient prop at cruise speed may not give adequate acceleration from low speed, or may exhibit nasty low speed handling traits. The result is often that a sub-optimum (in efficiency terms) prop is fitted, as this allows the best blend of handling, acceleration and adequate cruise efficiency.

 

Hi Powerabout,
for the propeller part, you can use the POP (Propeller Optimization Program) by Michigan university, which is based on Wageningen series. You can find the software here: http://www.boatdesign.net/forums/design-software/pop-ppp-35501.html . Outboard props are not really Wageningen series, but the difference is sufficiently small to make the program useful for most of practical purposes related to small boat design.

For the hull part, it is not so easy. There was an online applet for calculation of planing hulls with Savitsky's simplified method, at: http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html . But it's been quite for a while that it appears to have the server down. You can use Freeship's resistance calculators for evaluation of power requirements at various boat weights, but first you have to create a hull...

Hope that at least the prop part will be useful for you, if you know what numbers to put into it.

Cheers

 

Do the Freeship resistance calculators work for planing boats? As a canoer and kayaker I'm unlikely to find out . . .

 

Yes, it contains several methods for calculation of planing boats. Of course, one has to know what he's doing...

 

Jeremy,

I think that is true for some conditions and not for others. A jet boat works just as you say, by accelerating a mass of water back and developing thrust equal to the kinetic energy of the backwards moving water. This proven by the fact that a jet develops more thrust when releasing the water into air rather than into the water, which is opposite what many might think.

When a prop in the water is at maximum efficiency, the slip will be less than 5% so there is very little water being accelerated aft. According to many experts, the prop is then working more like a screw than accelerating water mass aft. So a prop running from zero to high speed is going from one of these modes to the other and everything in between. The lower the slip, the less the prop is acting like a jet and more like a screw. Large diameter propellers operate with lower slip than small ones and are thus more efficient.

Very powerful and fast race boats never operate in the most efficient mode and are much like jets. They trade power and efficiency for speed.

I'm sure a propeller expert can find things wrong with the above but I think it is essentially correct.

 

My only concern about my Tohatsu is the water pump. It is designed in a way that you have to dismount the whole base to change the impeller. Can't be done in the water.
This is the worst design I ever seen in any engine. I think all outboard are designed like that. This is piss poor thinking for a marine engine. The pump should be place in a way it is possible to change it in situ, without dismounting the whole half engine.
Beside that, it is the best engine I ever had. No electronics, old fashion 4 stroke, easy to maintain and repair (Beside the pump) and cheap to buy.

 

Sorry, but as a scientist (and prop designer) I have to say that this view, although widely held, isn't correct. The thrust from a prop comes from it changing the momentum of a column of water passing through it, by increasing it's velocity. If there is no increase in velocity then there is no thrust, and the boat stays still.

Many years ago (late 70's I think) I actually made some measurements of the velocity profile behind a single screw 80ft Clovelly class Fleet Tender, to validate a mathematical model that tried to predict prop performance more accurately. I don't have the data to hand, as it remained with my employer when I retired, but I do have a photo of me with the rig I designed which allowed a pitot tube to be placed at different depths and horizontal stations a set distance behind the prop whilst the vessel was under way.

The objective was to validate a mathematical model that predicted the velocity profile across the accelerated water column from the prop. I'm pleased to say the measurement system worked well and gave results that tallied well with the known hull resistance, but unfortunately my mathematical model proved to be lacking in fidelity (still, as you can see from the picture, I was still a very young scientist! (I'm the one in the dark blue jacket)).

 

Jeremy I sent you a PM.

 

- well there's the problem . . .

 

I thought the prop 50% sucks 50% pushes that still means it moves water aft