Hull modifications

you know, props, when a man builds a boat, he picks an engine, then fits a prop to the engine, he should in fact pick a boat, THEN pick a prop then last an engine and ratio to suit the prop

 

I never fished the Bay but I spent 15 years next door in Cook Inlet. A friend of mine had a Roberts 36, the Anna Marie. It was a nice boat. It had a single and did maybe 10 - 12 knots. It is a little bit unusual to see a twin. It is done sometimes, but most guys don't want another wheel to hang up a net in and like the more robust keel with a single. Boats like these are usually custom built to the owners specs. LOL, of course those Bay guys are crazy.

I've never seen one of these with tunnels before. The Bay has big flats, maybe they did it to get in shallower. Normally it would have a keel going all the way back with the shaft and prop exiting at the back of it with a steel plate tying the rudder to it . I realize it is a little too late but I think you are going to want some sort of keel set up on this boat. What I meant by trimming the skegs was cleaning up the net guards and sort of fairing things up.

It's only my opinion but I'm not sure I'd get rid of those keel coolers. I know they cause a bit of drag and don't look very clean but I never heard a guy ***** that they were slowing him down much and they are a really good low maintence way of cooling the motors.

I've spent a lot of time in boats similar to these and really like them. With a single screw and a keel I would feel safe in her up in the Gulf of Alaska in pretty miserable conditions.

I don't know where Roberts were built, probably here in Washington. I can ask around and find out most likely. There weren't a whole lot of them built I don't think but they were well respected boats. I've never heard anyone badmouth them and you certainly can't say that about all of these kinds of boats. The Anna Marie was a nice boat, well built and comfortable at sea.

Maybe I was a little optimistic about 20 knots but once you take the weight of the reel and rigging etc. off and optimise for speed it doesn't seem far fetched. Your v drive idea is interesting. I would have never thought about that. I'll have to ponder it. It might let you fill in those tunnels.

Have you given any consideration to a single screw? It would save considerable weight I think. Some guys like twins for the manueverability and reliability. I've always thought it would be easier and cheaper to keep one motor running reliabily rather than two. Half the filters, fluids, belts, starters, props, shafts, etc. With the hydraulics you already have you can install a damn nice bow thruster for lots cheaper than a motor and get back the manueverability factor. But I'm a single screw kind of guy, you should do what suits you. It is a thought though.

Also did you say what kind of gear you have? Many of these boats have Twin Discs which are Cadillac gears, very heavy duty. You could probably get away with a Velvet drive and save another big chunk of weight.

In any event keep us posted on your project and I really appreciate the pictures. Keep them coming

 

Wally,
Yes the boat has twin disc gears. I wouldn't trade them for the world though. -they're invincible!
I have thought a lot about converting to a single screw, mainly because of the bigger wheel that I can swing. I love the twins though. It handles great and the redundancy in a lifesaver. saved my *** once already when a fuel filter plugged. I drive a tugboat for a living so I am really partial to twin screw boats.

The Keel coolers are awesome but I am repowering to QSB 305s soon and these coolers are not big enough so back to the old heat exchanger I must go.

I think Roberts were made in mount vernon, wa. But I'm not positive.
Where is fox island?

 

I know what you mean about Twin Discs. I was just trying to shave some weight.

LOL, If you're working on a tug you don't need anybody's advice about manuevering boats or what constitutes quality equipment or good practice on a boat.

Mt. Vernon sounds right.

Plug these numbers into a plotter
Fox Island
47.20.315
122.34.624

My boat
47.20.315
122.34.624

Funny you should mention a plugged fuel filter. 3 days back I filled my boat with 100 gallons of newly mandated alcohal enhanced gas and a few miles out plugged my filter. I have twin filters with a valve so I can switch back and forth on the fly. A few miles further filter #2 plugs. Etc. Etc. Etc. Yesterday I swapped out the whole system of primary and secondary filters along with the screens at the carb. There was a rumor that this new fuel could raise havoc. I sure hope it resolves itself with this tank and only a few more filter changes. Having redundant filters makes this a bit less stressful but a motorboat without a motor isn't exactly what I call pleasure boating! Good luck with your project.

 

I used a load of biodiesel a while back. Lots of crud came floating down river for months. One night coming back in across the bar my port engine died. both the racor 900s and last chance filter was plugged. Made for a tense moment with the wind big swell and ripping ebb tide.

I only run clean reputable #2 now.

 

i see fox island. Very cool. I grew up in bellevue and had a bunch of friends in vaughn. do you ever go after those big hood canal spot prawns?

 

Every couple years I go after them. The season is so damn short it's not really worth gearing up. It sure was good in the old days though. So do folks in California ***** about people from Washington moving down there?

 

No we only complain about our neighbors to the south.

 

Hey, I had a second thought about your boat. You'll have to run this by some of the more knowledgable guys here but one sort of ******** hillbilly thing I've seen done on fat heavy bay boats to get them up and planing are adding big aggressive bilge keels, really more like gullwing lifting strakes.

Your boat is by no means a radical example of Bay Boat technology. Long ago the government limited the size of these boats to 32 feet. It was supposed to be a limitation so the natives wouldn't be too outclassed by the Seattle guys. Like all government ideas it didn't work out. The biggest bay boats are over 16' wide and 32 feet long with the nose cut abruptly off. They put 12 cylinder MAN motors in them and then a crew of 3 to 5 spends a short hellish summer chasing salmon in these overpowered abortions. You on the other hand have a fairly nice boat!

 

The chine on this hull was modified previously. It appears to have been more rounded. Glass over foam was added to create a harder chine and 4" splash rails up forward. The newer glass on this addition holds the only blisters on the boat. The blisters located from the chine up about 8" was odd. I ground out one completely to find foam and under the foam the original bottom.

I am committed to this project now. The keels are coming off completely and with longer shafts and rudders pushed aft.

Does anyone here have experience designing rudders? I am torn between building simple stainless steel plat rudders or ordering a set of bronze buck algonquin rudders. Is there much advantage of the foil shape over plate?

Is there a formulas for figuring rudder size? I am think something about 20 x 13.5...


this boat is once again keeping me up nights...

 

Others might disagree but I've seen plenty fabbed up out of stainless plate. Try this link:www.proboat-digital.com/proboat/200512/?pg=78

 

In terms of the steering force that a particular rudder can generate versus its drag when in line, a foil is substantially better than a rounded nose flat plate.

Taking a NACA0020 profile that is 15" deep and 10" long it will produce a maximum lift coefficient of 0.55. By comparison a flat plate of the same dimensions that is 1/2" thick will have a maximum lift coefficient of 0.23.

Does not look too bad - the foil seems to be only twice as good. Trouble is that is only part of the story where you are draft constrained. Once you start to increase the length of the flat blade the maximum Cd reduces. Lets say the blade is made 30" long but still 15" deep. The maximum Cl is now only 0.1. So although the area has doubled the peak steering force has actually reduced.

The actual situation depends on the available draft but usually a NACA 4-digit foil somewhere between 20 to 25% thickness usually gives close to the optimum result.

If you do not have a skeg to support the rudders then you will need a large shaft to take the bending loads. You need to check the bending but lets say you have a 2.5" shaft. A NACA0020 blade would be 12" long. Go as deep as you can while keeping the blade protected as much as you can by that deep entry. If it ever manages to rise on the plane then the props and rudder could be exposed to damage.

A flat blade will turn you but you are spending a lot of effort to reduce drag. Would be silly to add large flat plates for rudders. It is quite amazing how small the rudder area needs to be if you use a powerful section and keep the aspect ratio (depth/length) up around 1.5 or even higher.

The maximum force on the rudder in full lock at 20kts (say 10m.s) can be calculated as follows:

Force = 1025/2 * 10^2 * Area (sq.m) * Cl.

A NACA0020 rudder 12" long by 20" deep will give maximum force at 20kts of:

Force = 1025/2 *100 * 0.15 * 0.62 = 4766N

This is 1069lbf, say 1/2 ton. With two of them you get a full ton reefing the stern around at about 18 degrees of rudder. This is a lot to handle but then it is related to speed squared so drops off substantially as speed drops. Also it is unlikely you will need full rudder at 20kts.

There will also be a slight increase in water flow in the region of the props but with such large units the flow increase is negligible.

I should add that I do designs for high efficiency pedal powered boats. Here is my latest design in operation near Calgary:
http://www.adventuresofgreg.com/HPB/2008_06_29_archive.html
The rudder on this 24 foot boat is 2" long and 8" deep. It is a NACA0020 and is all that is needed to hold a course and do a circuit.

When I did my early analysis on rudders I looked around for engineering material and found analysis for ships that recommended foils with around 22% thickness for optimum performance.

Rick W.

 

Rick and Wally,

Thanks this is exactly what I was looking for. Great info! Wally that article from ProBoat was great. Rick, the depth of knowledge you have on the physics behind this is amazing. I must say it is very impressive.

My question for you Rick, is when I determine my rudder shape/size I will most likely match it to an existing production model. How do I know the shaft is of sufficient size? I had 1.25" dia shafts. Most production rudders I have found in my size range have 1.5 - 1.75" dia shafts? Are these sufficient? This boat is built big. Everything is built to receive an extreme punishment. I have kept with this theme with everything I have done, I want do not want to be second guessing my equipment at sea. I would like the rudders to withstand a "hard over" load at full speed with a standard safety factor. How to I insure stock rudder shafts are of sufficient strength?

Your calculations take into effect speed to determine rudder loads, but on a boat like mine wouldn't speed be lower while water velocity over the rudder being relatively high due to greater slip?

Second question is regarding rudder shape inside a tunnel. The previous rudders where close to square. Having the upper area of rudder surface inside the tunnels seems like it might be ineffective and possibly counterproductive, without and easy escape for the water in the tunnel other than in a downward direction when the rudder is at more extreme angles. Would my theory increase drag or effect steering forces in anyway?

 

I can't really comment intelligently but it almost sounds like you are describing a valve. But here is another link from some folks that probably could comment intelligently. If you download the brochures there are some interesting tables etc. that might shed some light on what is stout enough.

Remember the old fisherman's engineering maxim; if it won't bend it won't break

http://www.wbmetals.com/

 

A few things regarding the rudders.

You have removed the bottom support on the rudder. The V-strut would provide good support for the bottom. You now have to carry all the load through the top part of the shaft. This means a large bending moment. If you have 20" deep rudders and good foil sections you could generate 1/2T on the blade 10" down.

I know the engineering but I do not know the standards/codes. I am an engineer but boating is only a hobby. My paid work is to prevent insurance losses by looking at ways to design problems out. So i am very conservative.

I would be looking at safety factor of about 3 on bending proniding the shaft is not welded right at the top of the blade. I would go higher if it was welded. If the shaft ends up bigger than 2.5" I would want to look more closely at the detail.

So I support you approach to keep it robust.

As mentioned above these big proips do not slip much so water flow is close to boat speed once you get going.

The blade turning in the tunnel is not quite as bad as you might think. The main issue is loss of efficiency due to having to taper the top back of the blade so it swings without fouling. A blade influences water probably 2 to 3 ft either side of it. Getting rid of the big aft keels will make it more responsive but alos more skittish.

Rick W.