Displacement Speed Question

[apex1]

Originally Posted by ancient kayaker

There was a TV program a few months back that looked at highspeed projectiles entering water; by designing so the projectile traps and carries with it a bubble of air, it was found to travel a much greater distance and be less likely to break up on entry.

Years ago I have seen a report (maybe the same), coming to the conclusion, that this "projectile effect" could in theory make a submarine be propelled at speeds in the hundreds of kn.

Bearing in mind the power needed to pressurize the air, much more for a gas than for a liquid,

As I mentioned above, the Icebreakers did refit with pressurized water systems to gain a better effect at lower power consumption.

[ancient kayaker]

Actually I missed it at first read: thanks for bringing it to my attention.

[loups1]

hi rick.the engine reache easily the max rpm, the gearbox is a hurth with 2,15/1 ratio and yes the boat is in the water.[i reache the max.speed with about 3000 rpm-a little more than half thotle open]. i took the prop off last year and went to an prop expert who gave more pitch than the original. note that the same gearbox and the same prop was used with my former engine a lister-petter ac2w [11hp/3200rpm] and my speed was about 5mph.

[Rick Willoughby]

lou
Is the attached photo of your boat? You said yours did not have the trimming boards.

If not your boat, have you got a similar photo of yours out of the water?

If your engine is easily getting to 3000rpm and then continues to rev faster with no increase in speed it sounds like your prop is getting air.

The deadwood in front of the prop in the photo is not very good for going fast. There is potential for a bubble to form behind the deadwood. If this bubble is open to the surface it will feed air down into the prop continuously. You may be able to see a trail of bubbles in the flow off the prop if this is happening.

If it is the case then you need to fair the deadwood so the flow leaves as streamlines or stop the air finding its way down. The trim boards may actually be designed to do this.

It would be useful if you took some photos over the stern looking down at the propeller when you are at say 4, 5, 6 knots and also at full throttle. Video clips would be even better. They do not need to be very long just 5 to 10 seconds each so the degree of turbulence can be observed - preferably with sound as well.

You have heaps of power for such a small boat. It is a little overweight but not ridiculously heavy. It should not get to full speed at 3000rpm and then not go any faster eventhough the prop spins faster. There has to be a problem that you can observe.

Is the hull clean or badly fouled? That will make a big difference. Likewise with the prop. Is it clean?

My calculations show that the hull should get to 9 knots with 30 to 35HP so I expect you have a problem that can be identified.

Take some photos and video clips and post them here so we can take a closer look.

Listen for and slurping sounds near the prop. Look for swirls down the side of the hull just forward of the prop. Look for air bubbles in the stream behind the prop. Listen for a change in engine noise as you get near top speed and the torque reduces. All these things will indicate the prop is getting air. This is the most likely problem.

Have a look at the similar boats that go faster. Do they sit differently in the water. Have you got a lot of extra weight in the bow that is trimming the boat bow down.

Rick W

[loups1]

hi rick,thanks again.1] no the photo is not from my boat.2] mine has not trimming boards like this one but i put as i was advised lastyear infront and underneath of the prop 3]my boat is in the water but i'll try to take potos of all those you asked 4] no the bow is not overweighted because without the trimming boards which i put last year the bow in 4,5-5knt points the sky and after that speed all things from the bow come to the stern because it's straight up.[the stern almost is in the water.5] yes the prop is clean but the hull is almost a year in the water so now its not clean 6]i can not find any boats similar to mine in the water near me to take a look. 7] in your calculations how i can get 9knts, with what prop?

[Rick Willoughby]

The immersed boards will add more power than I have calculated but it is important to keep the boat in trim if you can. So the boards are doing the right thing for trim. The extra power they absorb is better than having the bow pointing skyward and the stern submerged.

What you need to look for is if the boards are pushing the prop near the surface so it is getting air.

The speed should increase as you increase shaft rpm. If it is not doing this then the prop is either loaded so heavily that it is stalling or getting air. Without knowing the prop size I cannot be sure the props are not stalling but it is unlikely. It is more likely that they are getting air or very disturbed flow.

The power calculation assumed a prop efficiency of 50%. This is quite low but realistic without having prop measurements.

Rick W

[apex1]

The "tabs" shown are intended to avoid air sucking by the props. They do nothing in terms of performance. Almost all of those Terhandri´s are way overpowered to be able fishing far enough outside the homeport. Yours is overpowered already, but if there are no "fins", "tabs" installed, sucks air into the prop. You could install 1000 hp with almost no gain in speed. Get a set of those fins and install them as every shipwright in Greece nows how and where. Then your power installed, will get you to a more than sufficient level of speed compared to power. Every step above is not cost effective, and you will not find it in any commercially used boat! 9kn are way above a sensible level for your boat and why should you believe to do better than those making a living with these boat day by day?

Regards
Richard

[Rick Willoughby]

Originally Posted by loups1

hi rick,thanks again.1] no the photo is not from my boat.2] mine has not trimming boards like this one but i put as i was advised lastyear infront and underneath of the prop 3]my boat is in the water but i'll try to take potos of all those you asked 4] no the bow is not overweighted because without the trimming boards which i put last year the bow in 4,5-5knt points the sky and after that speed all things from the bow come to the stern because it's straight up.[the stern almost is in the water.5] yes the prop is clean but the hull is almost a year in the water so now its not clean 6]i can not find any boats similar to mine in the water near me to take a look. 7] in your calculations how i can get 9knts, with what prop?

lou
Can you give me your best English translation of this:
"Δέχεται ένα κινητήρα έσω ντίζελ από 8 έως 35 ΗΡ και έχει maximum ταχύτητα με τη βοήθεια δύο παρατροπιδίων 9,5 knots."

I have attached the hull linesplan that I used for estimating the drag.

Using this and taking the initial data of 3000rpm, 2.15 gear ratio and 6 knots I can determine the existing prop is something like 12 X 12. It could be larger diameter and smaller pitch. Unlikely to be smaller diameter and larger pitch. It also assumes that at this speed the prop has not started to ventilate.

I have also shown the prop curve for engine speed of 4200rpm if it was working properly. It shows that the power rises considerably and expected top speed is 4.4m/s or just under 9kts.

The efficiency is lower than my earlier estimate so power is around 29kW or 39HP. Still within the capability of your big motor.

So you need to have a look at how the trimming boards work an other boats. Also confirm that your prop is sucking air in.

If you can confirm your prop size I can check the calculations.

The more information you collect the easier it is to validate the estimates and work out the best solution to get the speed you are trying to get.

Rick W

[Ilan Voyager]

I agree with you Apex, a part the "tabs" for getting a better efficiency of the propeller these hulls were never made for speed but for safety and ability to withstand choppy seas and the sudden gusts of wind so common in Mediterranean Sea.

I'm afraid that "improvised" modifications will adversely affect the safety of the boat, making it simply dangerous.

A 6.2m motor boat of 9 knots must have a very different hull that of these traditional boats.

[TollyWally]

Posting a bit late but squat boards were fairly common on old east coast fishboats converted with junkyard car motors looking for more speed out of displacement style hulls.