Stepped transoms

Easy, I don't know. I was designing a rudder for my boat and it came up with a friend of mine who had Jensen Maritime designing his rudder (and boat). He showed me notes from Jensen basically pooh-poohing what I thought I knew.
"They" say props (and rudders, to a degree) work by the Bernoulli effect, as well, yet I don't see a stream of fast-moving water in front of a prop and don't see very much of a suction on the low pressure side of a transom-hung rudder...
Everything I know beyond what I learned in fifth grade (we had a class where all we did was build a balsa airplane to learn flight theory) I have learned in the last few weeks.
It has always struck me that some planes handily fly inverted and it also strikes me that I often lose sight of the project getting caught-up in the academics!

 

Mark,
Bernouilli Sucks. The engine pushes the wing fwd. The air is pushed fwd and up by engine power (or gravity in glide). Inertia carries the air up further and a low pressure condition is created on the center and aft sections of the upper parts of the wing. This "vacuum" causes lift and lift is caused on the bottom surface of the wing sorta like a planing boat. One doesn't need a sack full of numbers and formulas to prove it or understand it. Bernouilli Sucks.

Easy

 

That's how my old man explained some of the basics of sailing to me. Getting sucked upwind.

 

Tolly,
I've flown quite a number of single surfaced wings whose "suck" held me up quite nicely .. ultralights and hang gliders!

Easy

 

Back to the question: why the "stepped transom." The more typical term is "notched transom" and you'll see it on most high performance boats. You'll see sort of "hacked" versions on many outboard boats, where they put the engines on brackets, sometimes with buoyancy, sometimes without.

The purpose is twofold: on an outboard or I/O boat, this puts the prop further aft, increasing the leverage of trim, so one can better raise the bow or suppress the bow as conditions warrant.

Of course, this reason does not work for the specific boat in the picture.

The second reason has to do with planing stability. A boat will porpoise AT SPEED if the center of gravity is too far forward from the aft end of the planing surface. Either move the weight aft, or move the aft end of the planing surface forward.

From displacement through the semi-planing speed range, you want just the opposite: center of gravity well forward of the aft end of the planing surface, to encourage planing at slower speeds.

 

The tunnel design has been with us for a long time and is not confined to small boats , The idea is to shallow the draft but it can also be quite efficent as well .
The design and the shape of the semi tunnel is very important plus all the setting up of the propeller and the placement , size and shape of the rudder .

 

Thanks for the replies. The question was motivated by a repower project. I have a 27' Farallon with an 250hp 8.2 Detroit that will be replaced by a 380hp Cummins QSB. The boat currently has a 17"x21" prop and 15 knot cruise fully loaded. I want to increase the prop dia. without going deeper in case I want to trailer it in the future. Hence the pictures of the prop pockets and notched transoms. I was curious if the notch would benefit my repower. I will post more details/ questions when I get back from vacation.

Thanks again , Tom

 

Going bigger could only add 1inch to how much more it will hang down and give you a 23 " prop . Things you have to worry about is clearance to the bottom of the boat ,or you will end up with vibration that you never had before and sound transfer from the tips of the prop as it will be to close to the boat .
Old story Change something and other thing change as well !!!change the prop dia , need to change the strut, change the strut you change the angle of the shaft , change angle need to tilt the motor and shift the stuffing box , you could just fit a longer shaft and get the prop back further but run the risk of bending the shaft and still ending up with yet more vibtation on your rudder and through to the wheel because the prop will be to close and if it goes back it gets deeper so instead of 1 inch it now turns into 1.5 inchs . sort of like chassing you tail so to speak !!!

Question I ask have you thought about what you are doing ?? do you really undersatnd all these thing ? simply by changing the prop from 21 to 23 ,all or any of these thing could happen . Why not simple add one inch to the pitch of the prop if its needed and problem solved . Are the gear ratios of the gear boxs the same on each motor ?or if its a straight drive 1 to 1 what about the revs of the engine ? is the power band the same on each motor or does one engine operate better at lower or higher revs ? gets confusing and a lot of thinking involved and at the end what have you gained ?? has it really been worth the sleepless nights ??

 

Flight delayed due to fog so I have some time.

Tunnels you're getting ahead of me. I do understand to big picture and how involved it will be. This is why I was asking why the vessels above had the notched transom in relation to the tunnels. Just the first of many questions.

The boat is heavy for it's length 27x10x 9,000lb so I would like to go to a bigger prop to enhance maneuverability and hopefully economy. The books tell us a bigger prop is better for a boat in this speed range. Possibly even a hull extension because I would like the engine out from under the cabin. Yes it gets even more involved than you thought!

You confused pitch for diameter, the currently prop is only 17" in dia. w/o enough clearance to go bigger, it has 21" of pitch. Also the current engine cruises and 2800 rpm new one will cruise at 2400. The 17" wheel would need 30" of pitch to absorb the extra HP with the same gear ratio- yes I could change that but I'm looking long term and want the most fuel efficient. Yes I could go four blade etc, other folks that have repowered these boats have all fell short on performance due to propeller size. One did move the rudder onto the transom and moved the prop back but were able to swing a 19" or 20" wheel if I recall correctly. They also tried some fancy four blade props. All compromises.

I'm looking at this as if I have a clean slate and can install the optimum drivetrain possible for the service I want the vessel to perform. I fish pots and troll for salmon so I think staying conventional prop shaft rather than the other drive systems available.

Off to try another plane outa here.

Tom

 

Tom,
I'd love to see some pics of your boat, especially the underside. I am very interested in your situation and your proposed solutions. I've seen more than a handful of stretched fishboats and most were pleased with the results. You are putting more forsight into your proposal than many who have just "stretched her a little bit for some more room or better trim." Happy Vacation.

 

Hi
Thats ok
I understand a little better now !
The Prop semi tunnels could be made as a unit complete with a flange round the perimeter and ready to install into your Fibreglass boat !Is it a glass boat ??
Simple a matter of Making the complete fibreglass unit on a bench and fitting the 2 legged strut complete with the gland at the front , the prop shaft and the rudder , Id be looking at twin rudders if you have to do lots a turning and getting close to pick up pots etc Means you could make the boat turn much quicker and become very responsive on the wheel eithr forward or backwards .
A tunnel shape for the 30"prop could even enhance the preformance in the way of top speed if you really do your home work on this !! And as for sucking the boat down at the back its counter acted by a sight turndown of the after end of the tunnel roof from the propeller aft . Its important to first understand the pros and cons and then design to suit your needs . The lead into the tunnels is very important as is the thing as a whole !
Heres my email address stuartwrcom@gmail.com
i have some pictures of a 147 ft boat with twin 3000 hp motors and it had semi tunnels on each side ! The shape of the tunnels was very efficent and kept the daught to a minimum . The Professional Boat builder Magazine had a whole artical on this very subject a couple or few years back and went through all the pluses and minuses !!
Have a look on "PROBOAT" and ask about the subject and ask to find the book and have a read of it !
As i have said it would be possible to simply make a unit complete to fit into the boat if you do you home work Right !. You have a time of the year when theres a off season so would be the time to do it !!
Now i understand a little more id like to help and follow what you would like to do . I will be off to china soon to Take up building and training staff to build 28 and 46 fot pleasure boats but wii always have my computer so keep in touch please .

 

Another canceled flight, this weather pattern should be over tomorrow.

Thanks for the interest in my project. It's a pretty big bite but should produce an ideal boat for my needs if it all comes together.

Thanks tunnels, I'll drop you a line and work on getting some pics of the hull posted.

The company was bought out and the cabin redesigned recently, mine is an older model now called the 2800.

link for Farallon: http://www.farallonboats.com/

A pic of a nice pocket design from a Backcove Yachts 29'.

http://www.backcoveyachts.com/backcove29/index.php

 

Thanks for the pictures of the drive I can pick up a few things from that
The strut need two legs not one , the shaft exit is not good and also where the rudder shaft comes down the need to be completely streamlined and faired so the is not water turbulance and distrubance befor or after the prop. The whole thing is to short and the rudder is to small and has little to no affect even when the prop is pushing . for your type of work Pro boat also had infor on Rudders a while before the one i mentioned . I strongly feel you would be better to have twin rudders Hydralic operated and linked together with the two arms facing aft not forward !! This is important all to do with the geometrics when they turn believe me It makes a differance same as the steering arms on you car ok !!
If the rudders are made properly they will do wonders for you steering going forward or back with out having to use lots of power to make things happen . Any way look forward to your future correspondance .

 

T & T,
I thought one of the advantages of a prop tunnel was a shallow shaft angle but this one seems to have missed the boat Ha Ha. That prop is almost going sideways!

Easy

 

shallow draught and shallow prop shaft angle are 2 of the advantages but if the tunnel has a efficent shape it also increases speed slightly as well . Its a trick subject to play with .