DIY tunnel drive

[Ad Hoc]

The problem with doing this, as with many aspects of design it crosses into another scenario. Doing this would make your boat very directionally stable...ie, and may become very hard to turn, which creates a problem for the rudder and its power and so on...but it all depends on how difficult it was to start with really.

See post #80, for explanation
Hull design for a small displacement boat

[CDK]

Originally Posted by Ad Hoc

The problem with doing this, as with many aspects of design it crosses into another scenario. Doing this would make your boat very directionally stable...ie, and may become very hard to turn, which creates a problem for the rudder and its power and so on...but it all depends on how difficult it was to start with really.

See post #80, for explanation
Hull design for a small displacement boat

It seems that I didn't express my thoughts correctly.
On both sides of the tunnels there are horizontal surfaces to reduce air entrance. I could extend these in the shape of triangles or squares until the edge of the transom, but not quite horizontal because the profile of the hull dictates the shape there.

[Frosty]

Originally Posted by Ad Hoc

The problem with doing this, as with many aspects of design it crosses into another scenario. Doing this would make your boat very directionally stable...ie, and may become very hard to turn, which creates a problem for the rudder and its power and so on...but it all depends on how difficult it was to start with really.

See post #80, for explanation
Hull design for a small displacement boat

12 pages of this now. Are we any where nearer than we were at page 2?

Ad Hoc said he could work out this problem with his dads pencil ( dont make me go look for it).

Your tunnels are too short and your under powered.

CDK you know that don't you, just read Ad hocs post he has'nt a clue.

[Ad Hoc]

CDK
I'd need to see a picture/sketch to see more, (plan and profile) before i comment, as not 100% clear, sorry.

[CDK]

Originally Posted by Ad Hoc

CDK
I'd need to see a picture/sketch to see more, (plan and profile) before i comment, as not 100% clear, sorry.

I used the word skirt and you thought of something hanging down. To me there is always an association with 'lady in red'.

Baeckmo says it is imperative to obtain maximum planing surface: two options are shown in yellow below.

[Ad Hoc]

CDK

A planing surface has length and breadth!...hence why i wanted to see how it looked in profile too, see how far it extends (aft) and how the transition is done up fwd, into the hull.

However, the lower figure left has view (the boxy one) looks best. But again, depends upon the profile and transition.

The extension can be made to work for you...ie have it as a storage for fenders and such like, as well as a nice diving platform too.

You just need to make sure you have good structural connectivity to the main hull.

[pistnbroke]

I thought I posted this before .KISS ..just turn the boost pressure up to 12/15 psi and up the fuel until it smokes ...thats all you are going to get ..after that you can fiddle with trim tab ideas.....

[CDK]

Originally Posted by baeckmo

With an average figure of kq=0.12 for this kind of prop, water density 1000 kg/m3, 1050 shaft rpm and a transmission efficiency of 94 %, we have a power consumption of 33 hp. This is for a prop in an unlimited volume of water; the flow restriction of the tunnel will increase power further, say at least 10 %. The resulting total developed power is then ~36 hp (~27 kW), which is very close to the preliminary engine data collected earlier!

Baeckmo, are you absolutely sure that a flow restriction before the prop increases the absorbed engine power?

[baeckmo]

Originally Posted by CDK

Baeckmo, are you absolutely sure that a flow restriction before the prop increases the absorbed engine power?

CDK, with increasing age, my inclination to give unconditional answers is reduced.......but generally yes, the propeller/impeller does not know what is causing the reduced inflow velocity!

There are two common exceptions though:

A) If the head (~pressure) loss due to the restriction is great enough to reduce the static pressure on the blade down to the vapour pressure of the fluid; then there will be cavitation. Roughly speaking: If vapour cavities are covering about 10% of the blade surface, you will have a performance reduction on conventional blade sections. Using cavitation criteria due to "Burril", your props if in free water (1050 rpm, zero advancing speed), would have about 6% cavitation. If the boost pressure of your engines had been lower, I would have suspected propeller cavitation.

B) If the disturbance is causing a pre-rotation, same way as prop rotation, there will be less work induced by the propeller, thus less power absorbed.

PS, Might mention that the power characteristics we see with axial impellers/propellers (increasing power with decreasing velocity) is reversed, when it comes to diagonal/centrifugal impellers. There we have increasing power with increasing flow!

[powerabout]

Baekmo
pretty simple when explained like that
Cheers

[pistnbroke]

if you put in all the power you have availabe and its till not on the plane then you know where the problem is ......god stop looking in the mirror KISS.

[CDK]

Is there any data or a rule of thumb about the wet area of a planing hull?

My estimate is, that under stable planing conditions the surface will be approx. 6 sq.m., so with 3.6 tonnes the load will be 600 kg/ sq.m.
The "trim plates" on the left side will be 0.25 sq.m. and get an evenly distributed vertical load of 150 kg. That applies to optimal conditions only.
For use at sea there will also be a dynamic load when planing through waves. Although most of the beating will be taken by the front of the planing hull surface, the trailing trim plates will also receive their portion, say another 150 kg.

In displacement, the plates will have no lift load, but there can still be a dynamic one either upward or downward in large waves.

With an assumed distributed load of 300 kg, half the load will be absorbed by the triangle formed between the trail end of the tunnel and the stern. But the other 150 kg is taken by the unsupported other half. If I construct the parts from 8mm GRP with a 50mm rib around the circumference and two diagonal ones it will be strong enough and doesn't need a diagonal support tube to the stern.

That is, if my assumptions are correct.

[baeckmo]

For pressure guessing: let's say we get her to 16 knots, ~8 m/s. That gives a stagnation pressure of 32 kPa, seen close to the spray root forward. At the aft end we see something like 30-40 % of this, acting at a trim angle of ~6 deg, ie a dynamic pressure of 1340 Pa normal to the surface. The flat portion of the bottom is operating at ~0.4 m draft, ie a static pressure of ~3920 Pa. In total 5260 Pa.

So your estimate, 6000 Pa looks reasonable enough in my eyes; including allowances for transients et c. And yes, make the square version, but give the chine edge a slight inwards taper (~5 deg) in top view, to avoid spray.

As said before, make the addition a continuous "stretch" of the bottom; no additional strakes, no horizontal or vertical strips, just an added bottom area inside and outside the tunnels! The inner tunnel sides must reach vertically down to the bottom surface. I would cut away the rudder balance in way of the vertical extension, so that there is a solid wall all the way to the rudder post. The outer t-sides should preferably meet bottom surface with 90 degrees, not vertically. This because the water is turning up into the tunnel with the bottom inclination as a base. Give edges bottom/tunnelside a generous radius.

[CDK]

Thank you Beackmo. I never got used to Pa instead of kgf, but I understand the reasoning.
Attaching the extra surface to the hull is something I still have to think about. In displacement and heavy seas there will also be downward forces due to banking and rolling. Probably the best is to laminate the plates to the full and the tunnels, embed a metal strip in the flange to the hull and add a row of screws just in case...

[powerabout]

Dont you just love the metric system...all these years and they still cant decide on a std.
fruit pascals and kilo grams per foot...what next?

Dont get me started on metric nuts and bolts...