Moderate Speed... Any Weather

I actually borrowed the man from someone else's drawing in the galleries.

The reason for being "on" the boat instead of in it is to increase under-deck clearance and thereby reduce slamming in rough water. I don't think the actual depth of the sides matters as long as railings of suitable height are provided. I intend to build a more elaborate cuddy and helm station after I'm sure I like the basic hull shape.

~ ALowell

 

I always wondered why tehere are no attempts in reducing slamming on cat's bridge decks. To me it would seem that almost anything would be better than the very flat deck - On my quick 3D sketch I have added small hull-lets on the deck - sure the hitting wave still can't go through the deck and will slam - however the impact should be much more gradual - and with more thought other better design should emerge.
Do people do this?

 

This kind of wave impact mitigation is common but doesn't really solve the problem. Even with a vee or vees, a wave eventually does not have anywhere to go and when it is trapped the resulting pressure increase causes a slam. What I want to test with models is the concept of starting the bridgedeck halfway back instead of at the bows as is usual in commercial designs. This reduces usable space but hopefully would give the bows time to rise allowing the deck to climb over the wave. Half-length bridgedecks are often used on bigger sailing and trawler catamarans, but as far as I know this idea hasn't been applied to planing cats of the size I'm interested in.

On a related note, I've heard people say that the reason Sea Sleds didn't have this problem was because they trapped water and air between the hulls, thereby making a compressible mixture and cushioning effect. Has anyone else ever heard of this theory?

~ ALowell

 

Alowell, if you are dead set on a power cat, here is a set of plans to look at, can be built from plywood. The previous page does have a write up about performance.

http://glen-l.com/designs/hankinson/wildcat-sport.html

But with that said, in my humble opinion they are going to be great in choppy water, but when it gets rough, and they have to slow down, I think they are going to ride just like a big pontoon.
That's where I say there are better designs for rough water in the semi-dissplacemnet hull area, that can still have a fair turn of speed.

 

I have been following this thread with a lot of interest. It seems to have settled on the power cat concept, maybe.
I thought I'd mention a hull shape that I saw some years ago that is not quite like any that have been mentioned. The boat was a Uniflite gill net boat that was fishing on Cook Inlet. I saw it before it was launched at the beginning of the season and thought to myself that it looked like a power robbing shape if I ever saw one. It was a 32' boat and the forward portion of the hull was very deeply veed and the fairbody line swept up very dramatically as it approached the area where the propellor and rudder were located in the stern, terminating in a very flat area there at the level of the chines. Someone told me it had a 230 hp engine. Later in the season I saw the boat running in some pretty sloppy weather and it ran beautifully and was making very good time, certainly much faster than I originally supposed it would go. Sorry I don't have hard data for you, but it impressed me a lot. I am not aware that Uniflite made any of their pleasure boats with a bottom shape like that.

 

If I went with a semi-displacement hull I would be interested in something like the Hereshoff power boat. Does anyone know where I can find lines drawings of his power designs?

~ ALowell

 

Gilbert, hull lines kinda like this.....

 

I thought maybe someone would ask for a drawing so I spent a little while playing with freeship this morning. I am quite confident that I have captured the shape below the chine reasonably accurately. Perhaps the chine and sheer were fuller in the bow and the transom corners may have been radiused but it's good enough to convey the basic idea. Freeship says the prismatic as drawn is .54 which may explain why I was fooled about how much power it would take to move it along.

 

Gilbert:

I've toyed with that idea myself and from what you said it sounds like it works well. The hull you drew takes advantage of lots of vee forward while still having flat aft sections to provide lift. Where it differes from most conventional designs is that the forefoot is deep enough to stay in the water at speed.

Do you think you could mail me a copy of that file? If so send a message and I'll get back to you.

Thanks,

~ALowell

 

The hull form that gilbert shows is a full dissplacement hull, and a 35 footer or so would scoot along at a little over 10 m.p.h. or so, depending on power.
Here is the same hull shape in a 24 footer, the noyo trawler by glen-l, it is a full dissplacement hull with a cruising speed of 7 and max of 9.
It seems that these conversations of fair speed in rough water always goes from all kinds and deviations of planning hulls, right back to rougher water capable full dissplacement hulls, but somehow or other the semi- hulls get ignored. Maybe there are so few of them, or they have taken a back seat to the planning hulls for so long, that most people don't fully understand or appreciate them for what they are and can do.

 

I am no stranger to estimating speeds of boats. It is my considered opinion that the Uniflite gillnetter was doiing over 20 mph when I saw it running in the bad conditions I mentioned. And I don't believe it was running at full throttle either. However, I could be wrong about both those things; but I bet I am closer to being correct than someone who didn't witness it. So it is not a full displacement hull form but at least a semidisplacement form. I would be very interested I hearing from someone who actually has one or had one or just knows exactly what it's performance parameters are.

 

I decided to upload the freeship file so anyone who is interested can take a look at it.

 

One more thing comes to mind as to estimating how fast this hull might go. A long time ago when I was first exposed to Crouch's formula I thought "What a useless, strange formula this is. It doesn't even take into consideration the length of the boat or any information about the shape of the bottom or anything else except the displacement!" So I never really used it in any calculations.
Some time after witnessing the underwater shape and the performance of the Uniflite gillnetter I still had no mental frame of reference to explain why it should perform so well. At some point, using an estimate of displacement, I tried Crouch's formula and it seemed to fit right in.
Using a mid range constant of 190 and the above freeship model's displacement and 85% of the 230 hp we arrive at 20 mph.

Crouch's formula
mph = C (constant between 180 and 200) divided by the square root of (displacement in pounds divided by 85% of the rated hp)

 

Gilbert, I took it for granted from your line drawings that the hull shape was a full dissplacement and according to length might squaek into the low teens or so.
But if it is actually a semi- dissplacement or semi- planning hull, which ever phrase one wishes to use, and if it was 36 ft. long (for example) it should easily do 18 knots or a little over 20 m.p.h. and maybe even more.
I believe the semi- hulls have a lot to offer in sea keeping and smooth solid riding boats, just at lower speeds, but still at least twice and more the speeds of the full dissplacement hulls.

 

Interesting Boat

I hope this thread isn't dead yet because I've found an interesting hull-speed boat that I think people might like to see.

This is called the "Allweather 26" and is designed for ultimate survivability and efficiency. According to the manufacturer it uses less than 1/2 gph at about 6 knots. The builder reports 12-20 mpg at reasonable speed. Among other unique features it has a keel-cooled engine and dry exhaust. These alone set it apart from any other production cruising boat and greatly improve reliability and simplicity. With 600 lbs cement ballast and a low CG, this boat achieves positive stability to 120 degrees.





One of the features I like about the boat - though it's something that could be incorporated into any boat - is the inboard-mounted bow railing that is typical of pilot boats. This allows a person to easily board other boats and step onto docks, etc.

If the Allweather really has 120 degrees of stability I would suppose it is close to self-righting. If it rolled would the cabin and windows survive or would they need to be reinforced?

~ ALowell