DIY tank testing.

[Frosty]

I was looking through the internet and found some information on 'vacuum break' I was very interested in this as it seemed to be one of -- or could be accountable for some of my problems . This was not a vacuum break like that of a toilet pipe or engine water injection but that of a break in the water as it leaves the transom of a planing vessel.

I pondered over this for some days untill it hit me to try a small model and a hose pipe. I found a piece of wood and made a model of various transoms on the wood. The piece of wood was about 1 foot and 2 x1/2. I made the perfect transom on one side and a slightly different transom on the other.

I then used a 12 mm hose pipe without sprinkler and with exellent solid flow and a pressure of say 50psi. I positioned this underneath the model and turned the water on. I was amazed at the difference of the wake leaving the models transoms and it certainly proved the point of the Vacuum break issued by Seafury surface propeller web site.

Altering the angle of the hose did very little, but this was kept constant at about 20 degrees. If I tried the rounded transom( whis was a sharp transom just lightly sanded to suggest the transom of my boat) the difference was immediately obvious and the water did raise up and into a rooster tail as apposed to the flat water of the sharp transom.

Although this may bring smiles to the professional tank tester, the results seemed very conclusive and the difference in wakes was very interesting.

Using surface propulsion I was always dissapointed at the confused water leaving the transom of my boat. This is not what surface props like. I shall be fitting a vacuum break on the next haul out.

Interestingly if a heavier rounded transom was used on the model. A radius of approx 2 foot (proportional to scale) the water boiled up the transom and never left a clean wake even under maximum flow the water would not break clear of the transom.

Does any one think that this simple experiment is valid and the results could be considered conclusive,-- or even usefull.

[Nomad]

I'm not really the guru on tank tests so I'll leave comments on that to the pros. As for the transom yes what your test showed you is true. The sharper transom will have a cleaner water break as it doesn't let the water ride up as a rounder transom would.

If you've ever looked at any highperformance boats you will notice that any water break on the hull is near razor sharp. The difference is quite noticeable.

[yipster]

didnt try the hose trick, but use cafee milk for fluid dynamics in the tub

[SamSam]

If a chine is the corner between the bottom and the sides, what's the corner between the bottom and the transom called? Sam

[Frosty]

Nomad, Thanks for your reply, this (for me any way) is very interesting and could turn out to be one of the more beneficial modification I have made to the boat. I noticed that you called it a 'water break' is that the proper term for it?

I have been searching the internet for more information using 'water break' but have not found a reall good source. What is the lip thing called that comes from the transom to break the water?

I have read however that an unclean break is a great drag for a planing vessel, Im not sure that I fall into that catagory, I just need to keep the rise of the water down to dry out the props more. The fact that my rooster tail off the props is very small is an indication that they are not happy.

[Frosty]

Originally Posted by yipster

didnt try the hose trick, but use cafee milk for fluid dynamics in the tub

Ah yes ,--submarine testing-- very interesting and much more fun.

[Nomad]

i'm sure there is a offical design name but right now it escapes me. I'll look in to it some more when I get a chance.

You wouldn't happen to have any pictures of your boat would you? What type of speeds are you running?