Tunnel drive performance issue

If the added plates don't extend below the hull and some distance forward of the transom, I don't think they will do much good. The bottom of the hull becomes the water surface when the transom clears on speeding up. The seal needs to go below this. It is not going to be a speedboat but I expect these fixes to make things much better. I don't see how the opening between the rudders and the added plates can be easily closed. If there is still ventilation, it will be on turns when the rudder turns and opens this gap further.

 

Thanks for putting useful ideas into words Tom.
I have something to keep myself busy this autumn.

 

Sorry, should have seen that you were posting from the Adriatic...

One option I have not seen mentioned (may have missed it) is to actually extend the driveline between the engine and prop. This will put your prop further out behind the boat and that alone should improve performance. You are then free to design a proper tunnel around the props if you still wish. You will likely need a hanger bearing on the lengthened shaft (any good machine shop should be able to build you a longer balanced shaft along with the bearing setup).

You may even decide to eventually build hull extensions out around the props, thus lengthening the boat and improving overall performance.

You might check out the tunnel designs by Atkins (e.g. - Rescue Minor). I thought of that when I was thinking of your current tunnel design as an extreme stepped hull (without the performance characteristics to make it function).

 

CDK
I would be considering an outboard jet drive. If you look at the intake of a jet boat the flow is quite torturous. I believe you could achieve streamline feed into the prop.

So you would fully enclose the prop with a tube. A slot would be cut from the bottom of the tube right at the transom to a few inches from the prop. The slot would be the full width of the tube so a big hole in it.

You then form skirts from the slot to just below the line of the transom along both sides and the back edge of the slot.

There would be a fairing plate running from the transom to the top of the tube inside the tube. The rear skirt would be angled to scoop or jet water cleanly into the prop.

The system should exclude air and create very nice nozzles for the prop. The best steering would be achieved with full flow vectoring nozzles pivoted on the end of the fixed nozzles like standard jet nozzles. Rudders in the stream would not be all that effective. A rudder off the transom would increase draft and would not be very effective at low speed but then you do have two engines.

Here is an example of a jet that shows the flow pattern that you would like to achieve:
http://www.hamiltonjet.co.nz/hamiltonjet_waterjet/waterjet_working
You need to keep as much open area ahead of the prop and even neck it down a bit at the exit to adjust for the velocity change over the prop. If the inlet is necked it would create negative pressure and just suck in air. You have a large prop so the velocity ratio would be much lower than the jet shown so slight necking is all that is necessary to get the best condition. You may find the props actually bog the engines if they really bite well.

You should be able to make quite an efficient low pressure jet. Effectively a partially exposed Kort nozzle. With the big shrouded props the efficiency will be high. It will offset any extra drag from the cowling by a good margin.

The cowling would need to be solid but having a nominally tubular form will add strength. Could do a trial with 1/4" plate on one engine and go from there based on observations. You may find a length of pipe having suitable diameter to start with.

Rick W

 

Rick, I am familiar with the Hamilton site, but nevertheless thank you.
Before I started converting my boat I read everything I could find about
jets. Subsequently I started believing at least that much of what I'd
read, that early 2006 my boat looked like the attached picture and the
nightmare begun.
That same year I decided to admit defeat. I removed what was left of the
jets (and believe me, it wasn't much) and started constructing from
scratch.

And now you use that dreadful word 'jet' again... But at least you've
added 'low pressure'.

I will seriously think about your suggestions, but the implication would
be another winter spent changing and constructing. That my life would be
made easier if I would stumble upon a length of thick-walled stainless

tube with the right diameter, sure, but the chance that I do is very
close to zero.

I also fear that if there is no ventilation at all the 14x17" props will
require more power than the diesels can provide so I'll have to replace
them.

CDK

 

If you want my suggestion, I think you should also:

1- move the rudders BEHIND the props
2- extend the tunnel sides aft past the ends of the props by a foot or so
3- extend the tunnel sides forward at least 3-4 feet and maybe as much as 7-8 feet
4- extend the tunnel sides down to several inches below the bottoms of the props / rudders

The too-short tunnels -- way too short in both length and depth -- are primarily responsible for letting in air, but the air problem will never be resolved unless you also move the rudders back behind the props where they belong.

 

CDK
If you do not want to get to the Kort nozzle type enclosure - lets say full shroud, then the other option to increase performance is to get some proper surface props.

I have no expertise in their design but your observations elsewhere and my observations and experience at low power are not far apart.

If you have multi-blade with EAR of 1 then you at least dramatically diminish the vibration over what you have now. The blades do not act as foils as the ones you have are intended to do. I believe the best have knife sharp leading and trailing edges. The front of the blade tends to work in a partial vacuum to reduce friction. The leading edge of each blade has progressive entry while the trailing edge has abrupt exit.

The blade loading on surface props is going to be awful because the blades are fully loaded and unloaded each rev but this is not a lot different to big props on heavily inclined shafts. I have actually bent shafts through induced uneven loading on inclined two bladed prop. I used to wonder what caused the vibration until I bent a shaft and was forced to understand the physics to resolve the problem. So many fully submerged prop blades do not see nice even loading either.

The third option is to go boating (at steady pace) and let it niggle away at you till you are re-energised enough to take on the next phase of development.

Certainly a pair of purpose made surface props is the least effort path but they are no doubt expensive. Actually I was surprised at the low cost of some of the racing ones referenced on this forum. They were an order of magnitude lower in cost than I expected but much smaller than you need.

Rick W

 

That is what we've been doing this season and if the weather holds, we keep going until the end of September.

Please look at: http://www.performanceimports.co.nz/saro.html
Saro claims that their tunnel is solid water filled although the transom part is closing off the tunnel entrance like with my design. Their drawings shows something the I interpret to be a surface piercing prop, but the photo shows a normal one and the text confirms that. The fat blades are not what you would expect there.
Is it bogus or could the peculiar tunnel shape with a large obstruction just ahead of the prop really do miracles?

CDK

 

That is not too different to what I was describing other than I believe you would get benefit from a complete shroud around the bottom part as well. It would be much simpler to make than I have described but achieves the important result of cutting off the air into the stream.

If you could replicate this I believe you would be on a winner. The main points are to seal off the air with side skirts and ensure enough open area ahead of the prop to avoid a low pressure area that will suck air in under the skirt.

Eliminating the complete shroud reduces the concern you have for the prop getting too much bite, ay least a little. In any event if you had to reduce prop pitch it means you have gained efficiency which is the main point.

Rick W

 

When I read the claims made by Saro...I scratch my head. Is it a surface propeller drive or not? Some affirmations by Saro are a bit simplistic. I wouldn't take the Saro claims as technical basis. ****! no more euphemisms: to be clear it is commercial crap. I won't analyse here the claims but they contradict themselves in their explanations and if it works as they say the Saro drive becomes the Saro brake-drag drive when fully down with its enormous wetted surface, or I've spent 4 years in a naval engineering college for nothing. And going from 45 to 55 knots (size of the boat? displacement? HP? what drive first used? propeller?) with a new drive I would like to see, or the former one was a true piece of ****. Sorry for the language but such commercial crap gets me angry. Lets cool down.

Surface propellers are very efficient, and do not vibrate when well designed.

In the eighties we made trials of the proto of a small patrol boat (60 feet), a kind of modern PT boat, with "ordinary" propellers, water jets and fixed surface propellers, using the same engines.
The surface propellers were clear winners: smooth, better acceleration and by no means the fastest and the most efficient. They were the easiest to tune as they did not cavitate (cavitation and aeration are 2 very different things), and when too charged they slipped, so they acted a bit like a torque converter. They needed just to be well fed in air and exhaust gases.
Waterjets were the slowests and hardest to tune (that doesn't mean that waterjets are bad, simply they were not good for this application), and the normal propellers gave the habitual problems (resolved) of vibrations, resonances and cavitation at the beginning.

It's very tricky to design a tunnel in a hull for the propeller. Very difficult to get a good flow at every speed on a planing boat. A lot of naval engineers look at that with suspicion. Myself I think of tunnels as a bunch of troubles and headaches and stay away.

When I saw the pics of your boat I thought at fist glance: fixed surface props...

You have received some good advices; go to "aerated" propellers with big cups or surface propellers. You can ask France Helices, or some italian firms to help you. It will be the cheapest (I didn't meant inexpensive) and easiest way to solve your problem.

The propellers are not deep enough and you'll have always aeration problems. The extensions won't do any good and at some moment the propellers will start to suck air. So why to fight that? it's not a failure just switch to propellers able to cope with aeration. You'll be surprised by the good results.

Cheers

 

I had the same reaction. Their claims are full of contradictions. I don't have either experience or book knowledge of surface props so can't comment on them. The original wish was to get the boat to run in the teens, so are surface props good for that? I do think the cheapest answer is to cut down on the ventilation by adding some depth to the skirt. Next thing might be to move the rudders since they contribute to ventilation.

 

CDK has managed to make a surface drive

Surface drives are not only for speed, race or dragster boats. It's the simplest way to have a low draft boat for beaching. Yes I know, Arneson and others claim speed and etc and their complicated drives are designed for that, for a market wanting always more speed and sophistication. So surface drives are seen as for speed. It's a false statement, result of 30 years of marketing.

Xrudi (http://www.levidrives.com) has one cheap in galvanised steel designed by Renato Levi for small pro fishing boats in Malaysia as excellent alternative to gas suckers 2 stroke outboards. Small fishing boats are not racers, they go in the mid tens knots.

CDK has managed to make a surface drive and I wouldn't lose all the hard job done, and I saw a few as simply made having good results.

When you see the pics and drawings, you identify the hull as a planing one. It behaves differently following its Froud Number (or coefficient of speed if you want):
Slow speed; displacement mode and the water has the time to follow the hull keeping the propellers under water. (It's the bad time for a surface propeller you have to give it air and/or gas exhausts).

No problem until here with normal propellers. Acceleration and the boat begins to lift the bow, the water flow starts to break from the bottom of the hull at the stern. It's the transition: the top of the propellers is too close from the surface and the propellers start to suck air (the famous white foam described accurately in the first post by CDK).

When the boat tries to get out of the hole, the water flow is unable to fill behind the stern (it's normal the hull has been designed for that) and makes a "hole", the propellers loose grip, and start to hammer in the water (the vibrations) being unable to push the boat which stays his hole.

Look at the pics and drawings the propellers centers are just above the bottom of the hull. Practically the design of a surface drive. If you allow the boat to plane and the flow water to break cleanly from the stern, just one half of the propeller will be in the water.

Whatever you put over the propellers they will suck air, or you have to put the propellers in green water under the line of the bottom of the hull with a antiaeration plate. This solution means to destroy all the work done.

So why not to go to surface propellers?? and aerate them with the exhausts?

 

Modifications

For the next phase of this project I took the boat out, removed the props, stern tube supports and cleaned all relevant parts with hydrochloric acid. Since fairing of the transom's edge was no option because the mechanical parts both inside and outside are located there, I made elliptical discs, warped into an S-shape, from 4 layers of GRP and glued these in the tunnels using roving and epoxy. The cavity behind these I overfilled with polyurethane foam.
It still is not the ideal shape for a flawless flow of water at higher speeds, but a vast improvement compared to the sharp angles that were there before.

The outside tunnel edges now have skirts as a barrier against air that was drawn into the tunnels. The gap between the tunnels is closed with a stainless steel plate until the point where the rudders are, also to reduce the amount of air within the tunnels.
The vertical plates, as suggested by tom28571, I did not install, although I gave it a lot of thought.
Vertical plates near the stern are in fact rudders, and adding fixed rudders with more surface than the movable ones will certainly reduce their efficiency to the point where it will not be possible to steer when running on one engine only. In fact, the last time I approached the slip on one engine, two attempts failed because of gusty crosswinds and sluggish steering response.

After a new coat of anti-fouling for the hull and spray coats of the far too expensive special stuff for props and rudders (from Int'l Paint) the boat is back at its berth now, waiting for inevitable test run when the weather permits it.

A lesson I learned this time, is that stainless steel gets much, much more hard to remove marine growth than gel-coat or paint and the zinc anodes are eaten faster if more steel is exposed. So I now painted all steel parts with a primer and copper based anti-fouling.

The white conical part around the prop shaft is a nylon cylinder that rotates freely around it. Its purpose is to prevent fishing lines etc. from wrapping around the shaft and causing damage. I found this to be very effective, but the disadvantage is that a simple prop puller cannot be used, so I had to make a 3-legged tool for that job.

 

Good news and bad...
Much, much behind schedule because of damage, weather and other matters needing my attention, I finally made some test runs with the improved tunnel entrances.

The good news is that steering response has improved dramatically. Except when mooring in a cramped space - which still goes best by playing with the engine rpm and fwd-reverse- it is not necessary to touch the throttle controls. The 4 small rudders prove very capable to change direction, both on one engine and both.
Also the violently agitated white wake has gone, so the flow in and behind the tunnels is much better now.

But the bad news is, that the speed has gone down from 14 knots during the try out last year to no more than 10 knots now.
The engine rpm at full throttle barely reaches 2800, so the engines cannot produce more than 60 pct of the horses I expected.
This is certainly caused by the better prop efficiency now that there is no more air in the tunnels, but the result is more than a little bit disappointing.

Imho there are 3 ways to improve the situation, and I'll have to make the right decision:

1. Increase the engine torque by modification of the turbo chargers or the waste gate valve. Not good for the engines, but I probably just need it to reach planing speed.

2. Invest in 2 new props with less pitch or a smaller diameter.

3. Somehow introduce a controlled amount of air in the tunnels again.

Any bright ideas are very very welcome!

 

Go the least expensive way first - change the props. At least you should manage to make the engines rev at the correct rpms and deliver the power they should.
But I'm affraid that your vibration problems will not disappear. You have two hydrodynamic brakes over there at the transom. The waterflow just can't follow the path you are trying to impose on it.
You will have a very strong recirculating vortex attached to the transom in front of your prop, creating a huge drag and strongly disturbing the flow seen by prop blades.
No satisfactory solution for your problems can exist, imho, if you don't manage to eliminate that sharp edge in the tunnel, at the bottom of the transom.