Planing Power Cat plan and hull type question

Totally agreed !!
[I This design was born from the need for speed and race boats , thats were they are at there best !!going fast !![/U][/I]The height above the water of the tunnel and the way it compresses the air between the two hulls lifts and holds the boat in surface effect !
Have you heard about the term surface effect ??

In choppy water they are like nothing you have been in before !!! the worst thing drivers can do is ease off the throttle !!
Thats the time you give it all its got trim to its optimum and hold on . The boats are designed to fly!!! litterally .

Cats are on the other side of the coin and the hulls and configeration do not work the same way . Two hulls ,two motors ,and a big gap between (high above the water tunnel roof ) the hulls are normally identical in shape but some are not . They are stable plateforms for fishing , ride well in seas and do not need to be 100 % aerodynamic .
Althought they can and will travel fast given enough horse power and balanced properly they do not behave the same as a tunnel hulled boat !!. The tunnel of a cat does not perform the same function as a true asymmetrical tunnel hull boat !!
Know and fully understand what you are looking at !!
Its important also while you are just looking that you come to grips with the major differances between the two differant boat designs .
You need some seat time in both !!! if you dont know much about either then riding and driveing in both in the same sea conditions .
Reading and trying to come to grips with whats good and whats not can only come from first hand exsperiance and getting out there and doing it !!
What you like and how you drive will not be the same as what the other guys does so it then becomes personal .Remember also about weight and balance in either of these hull forms .
Weight kills speed !! and in the wrong place can really upset balance and stability and safety at sea !!.

Me i am a true and tried Tunnel person hence my name TUNNELS .

 

OK Tunnels you're a tunnel hull man, any successful ones in the size range and intended usage mentioned above you can show us ?

 

Based on what jcros is looking for, you are right. But I'm glad that Richard put Skoota into the mix for the extreme fuel efficient option.

 

The 26 is around 4500lbs, the 24 is about 3800. But I am pulling this from an admittedly poor memory . I know the 24 was 4000 including the duel axel trailer without fuel, for shipping weight but I can't remember the exact specs off hand, and Twin-Vee has redesigned the boats too much to reliably pull the data from them now.

Their boats are true planing hull cats,with a reasonable high clearance in the tunnel, and only minor sneezing. Heck, I loved the 26 so much I bought the 24 as a dive boat for down in Jamaica where it routinely run 20 miles out with 5 divers and 8 tanks at 30kn or so. Into the open Carribean sea (headed mostly south from the south coast of Jaimaica.

I am looking for my fuel flow charts for the boats. If I can find them I'll put them up later today.

 

Used to work with and for Scott Robson and we made a production 5 mtr Tunnel Called a KONA with a pair of 55hp suzuki and used to go well ,the same boat with a pair of Yamaha 60s was really good ,and with a pair of 70hp omcs was boardering on dangerous .
The 22 footer we were making had 2X 140 hp Yamahas and went good but i always felt the 175 hp twins set up would have been the ultimate . At that time Scott had a 30 footer with a pair of 200 yamahas and that could almost fly . All these boats in choppy water were so comfortable to ride in even the little 5 mtr boat .
We took a 5 mtr out in the open sea and played for hours jumping waves just like a jet ski . Even a novice boatie in less than 5 minutes was jumping waves !! Fantastic !! Ever since then i was hooked .In a moderate sea it was possible to simply set the boat going at a comfortable speed and not touch the wheel and it would track in what ever dirrection you set it at . In the calm water of a harbour the same stand on one side and it would slowly turn a little stand on the oher side and it would come back again . BALANCE !!
TUNNELS DONE AND SET UP PROPLERLY ARE EXSTREMELY USER FRIENDLY even a 10 year old kid can drive one safely !! The 5 mtr was a family boat after all !!



Scott Robson power boats . tunnels not cats !!!!
Last i heard he was still based in Christchurch New Zealand , possibly has a website ! www.robsondesign.co.nz/ - Found this but the site is underrepair . Has a boat called a stealth 22 thats worth a look at !!

 

That probably is on the light side here, I know a popular 23 footer here that could easily go 4 tons over the weighbridge on trailer, they weren't made to break.

 

I had trim tabs on a V hull and used them often while people moved about but I never noticed any fuel efficiency, I didn't notice fuel efficiency at all, just filled up. Do tabs help with fuel efficiency?

 

Yes. Boats are at optimum trim on an even keel and at a particular bow up angle. Due to loading and conditions like stuff and people a vessel may not be in optimum trim. People may also move around while the vessel is underway.

Trim tabs allow the operator to adjust trim while underway and to keep the boat trimmed properly. They can also increase fuel efficiency and range by allowing a vessel to stay on a comfortable plane at a slower speed than it might be able to without tabs. This allows the vessel to travel on plane at a lower throttle setting.

Boats float and move fine without trim tabs, they don't "need" them. But they will get on plane faster, stay on plane at slower speeds, and stay at trim regardless of changes in load and conditions with trim tabs with the touch of a button.

Steve

 

Thanks, I thought so, but had not really noticed, but that was when fuel was cheaper...lol. Then what would the effect of tabs be on a cat? Not to get OT, but if it was a sailing cat modified as a motor, would the tabs make up for the rocker of the sailing hulls? Design longer or wider tabs?

 

a few quick points

I try to design efficient boats, not just boat that float and move along. So a sailboat hull can never be a good motorboat hull, or vice versa of course

So for this application I would try to maximise the speed/hp ratio rather than try for maximum speed

My power cats have totally different hulls from my sailing cats

A 20ft geosim of the 24fter you mention will have less than 60% of the displacement of the 24. So it can never have the same size engines

I have written it many times:

A planing boat needs a lifting surface, so to be efficient needs a high AR. A powercat hull has a very low AR, typically say 10:1. So it can never be an efficient planing boat UNLESS it develops lift in some other way, most obviously by using the air lift from under the bridgedeck. But that only sensibly works at high speeds.

And after this debate, what does the OP think now?

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Tunnel boat design

jcros - check out this web site.

 

i agree, i like richards designs too and one day i will change over to an easily driven hull myself.

 

I see pics of the 22 on the net and it's not what I'd call an offshore boat. Think bar crossings might be hairy in that one.

 

Yeah i totally agree ! Its a scream machine . But the tunnel thing is a beautiful ride in the chop like nothings i have ever experienced before and the ease of driving !!. Tunnels don't have to have twin motors all the time . they go just as well with a single motor The same 5 mtr hull was also a open center console boat as well and with a single Yamaha 90 was a pleasure to drive and very economical as a fishing going far away places boat .Still had the same twin fuel tanks so had a long range for fishing and just cruising all day long !!I have some pictures buried some where must have a look see and see what i can find !!

 

Range of Deadrise Angles

Very good!
By the way do you know what is the typical or preferable range of the Deadrise Angles in degrees (for symmetrical hulls)?
Let's assume speeds 20-30 knots, V hull short power cats of 9 to 12 meters, offshore cruising, a less rough sea and the minimum requirements of comfortable riding.
I also heard that variable deadrise mid to aft is not much recommended.
Thanks
APP


{PS: It seems a good compromise could be a 20 to 25 degr. deadrise}

Added: 5-Dec-2011
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From a couple of articles about deadrise
1) From: http://www.navaldesign.co.za/articles/Planing Hulls 07 Oct 2006.pdf

2. Deadrise
Deadrise is the angle a hull bottom makes with the horizontal plane viewed from ahead or astern. The right amount of deadrise gives a boat directional stability, a softer ride and reduces wetted surface drag as the boat rises on a plane. Deadrise is said to be “constant” if it stays approximately the same from midships to the transom. Deadrise is “variable” if it changes from a deep angle at midships to a shallow angle at the transom.
For inshore crafts, deadrise can be about 10 – 12 degrees from the midships aft, increasing from midships as you go forward towards the bow.
For coastal craft, deadrise should be 15 to 20 degrees from midships aft, increasing as you go fwd towards the bow.
For offshore boats, deadrise should be 20 to 25 degrees from midships aft, increasing as you go forward. Some very high speed offshore boats use deadrise in the afterbody as high as 26 – 30 degrees. This is to soften the impact of reentry when the entire boats jumps clear of the water and slams back down, at speeds in excess of 50 knots
In general, the deadrise angle determines at what speed and seastate a planing boat can best power.

2) From: http://www.discoverboating.com/newsroom/faqs-advice.aspx?id=11&qid=57
Deadrise is the amount of V-shape on the bottom of a boat’s hull, measured in angles at the transom of planing powerboats. In general terms the larger the degree of deadrise the more V-shape of the hull, so a flat bottom boat will have a deadrise of 0 degrees, while a boat with 24 degrees of deadrise will have a deep V hull.

In terms of performance the greater the V-shape the better the boat will slice through the waves. The shallower the V the more it will push over them. So a big deadrise is better for rough water conditions and a small deadrise is better for smooth water. The more a boat slices through the waves the more energy it will use. Thus boats designed for rough waters have a greater degree of deadrise and are slower. Boats designed for smooth water have a lower degree of deadrise and are faster. For example, a ski boat designed for speed and water sports, used on a lake will have between 10 and 17 degrees of deadrise. On the other hand, an offshore sportfishing boat used on the gulf stream off the coast of Florida will have a deadrise greater than 20 degrees.