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Been a while since I posted a progress report on the 8 meter cat I'm building.
We're welded out on the inside, outside seams are grooved out to remove contamination and we're about to start the outside sealwelds. For this grooving we used a new style saw from Sears, a twinblade. This is an unusual tool, it uses two counter rotating blades about 6". The saw has no shoe, it's entirely freehand which is really weird for someone used to using a skilsaw. But it has absolutely no kickback, you can actually pull the saw backwards while in the cut. Noisy sucker but very impressive.
We held off construction for about three weeks while we tried to reach a decision on power. We had planned on using the little mercruiser 1.7 liter diesels and outdrives but the performance reports I'm getting on similar installations are not encouraging, it appears that this really is too much load for the engines. The new D3 volvo looks like a good candidate when it comes available, it's 160 hp and about the same weight. Unfortunately it looks like it'll be a while before they're commonly available. So, we've made the painful decision to go with outboard power for now anyway. I really had my heart set on diesel power, dangit.
But we can always repower if and when the right engines come available and we're going ahead and finishing the topsides off as designed for inboards, with a full width transom platform. I'm talking with the designer about a couple of hull pod extensions to go under the platform to allow mounting the outboards on the edge of the platform. I'd appreciate some advice on this from you guys who have used brackets...
I know I want flotation from the brackets to counteract the weight of the engines and to help with following sea conditions. Most of the brackets I see, including the kind that Warthog is installing on his project seem to have an angled bottom and allow mounting the outboard cavitation plate considerably higher than the hull bottom, I guess the water swells up aft of the transom to permit this. How much higher can the engines be? Since the bottom edge of the bracket angles up wouldn't this mean that it's really not providing any support while you're on plane, maybe making a tendency to porpise? Would we be better off to make the bracket the same angle as the hull but inset from hull edges? I guess I'm thinking that the reason brackets such as Armstrong have a flat angled bottom, rather than a V is so they can fit in a universal fashion on many models of boats.
Since I'm going to build this myself rather than buy from a manufacturer I don't want to call and pick their brains. I would appreciate any thoughts on this so I can state a preference to my designer.
Will post more pictures when they show someting significantly different than the last ones do. If all goes well this week we'll be turning her over next weekend which should be an interesting event. Thanks for the help.
I saw the Craftsman Twinblade on a NASCAR preview last night. Looks like an amazing tool !
I know you've had your heart on diesel power, but I expect you will save big $$$ on installing outboards. Unless you plan on putting huge amounts of hours on your new baby per year, I suspect it will take many, many years before the lower cost of operation of the diesels will overcome the lower cost of installation of the outboards.
4 stroke or DFI ? Don't waste your money on any of the "old" tech carb or EFI 2 strokes ! Got a brand in mind ? I lean toward Evinrude, but the new E-Tec technology won't make it to their V4 and V6 eninges for another year. A pair of Johnson/Suzuki 140's would be good too !
You have some good questions about "pod extension/brackets".
I'm sure you realize this already, but the floatation won't help much when the boats "at rest". An air tight chamber works just as well, and with an inspection cover, you can see what is going on inside (hopefully, nothing). See what you can do about moving as much weight as possible forward.
Second, be careful about properly sealing any holes, including the inspection cover. Lot's of folks on THT with "water in my bracket ... how did it get there ?"
I'm no expert, but I would think that the pod should have the same angle as the running surface of the hull. If this is truly a planing hull, then the cavitation plate should be slightly below this line. What does your designer say ? They should be the expert !
Another thing to think about is the waterline "at rest" and what length outboard leg you planning on. I would go for a 30", just to keep the engine high and dry (no difference in prices for a 30" vs a 25" at Ed's Marine )
Oh I guess I like the mercs, the've got some good specs but the 116 prop hp is the problem. As I understand it it's enough hp for light loads, smooth conditions but loaded and heavy seas is another story. Interestingly, the cost is about the same as outboards but I don't want to own an underpowered boat.
I like what I hear about the Johnzuki 140, it'll either be a pair of these or the new Yamaha 150. Definitely leaning toward 4 strokes. Significant price difference between the two I wonder if the extra 10 ponies is worth it. From what I hear around here, though, the Suzukis are somewhat overrated on hp while oppo is true for Yam.
My welded cat w/twin o/b's doesn't have brackets or extension pods. The engines are mounted on jacks at the transom. I'm not sure I would want to hang heavy engines way out back on a cat unless your pod provides the full displacement of the hull to the mounting points. Jacks are cool because you can really trim for conditions much better than if you only have engine trim plus, in your case, engine bolt-up to the transom is simplified. When you repower just take off the jacks and punch through for your outdrives. The swim step would have to wait though or create it like on this one by virtue of the aft cockpit bulkhead being moved forward to allow full raising of the engines. When you repower shift the bulkhead back and add your swimstep. Just some thoughts.
The photo below also illustrates what happens when the builder gets in a hurry and does a lousy bottom paint job (which has become a whole 'nuther issue on alum hulls)
It's good to get an update on the progress that you are making. Please post some pics when you get a chance.
I replied to you a few months ago on another thread regarding your alloy cat. I thinking about building one and I would like to talk with you. Could you send me an email (joemaj888@hotmail.com)
IRRC, NMMA allows +/- 10% on advertised HP rating. Your comments on the Johnson/Suzuki being a bit "overrated" may be true seeing as this engine started out as a 115 hp. The Yamaha 150 may be underrated a bit as rumor is it will be "growing" to 175 within a year.
The Yamaha 150 4s is about 50 lbs heavier than the Suzuki 140 4s. The Evinrude 150 Ficht is about the same weight as the Suzuki.
WOW the Johnson 140 is $2500 less than the Yamaha 150 at Ed's
Chaps, that looks like a lot of oxidation on your hull ! Over what period of time did it occur ? I would hope that paint is not your only protection. Do you think you need more/bigger zincs ?
But hey, you Great Lakes guys don't have to deal with 'em so be happy. All you're seein' there is bottom paint failure & barnacles. Zincs? I've got big honkin' zincs and they've been working overtime with all the exposed bare metal underwater.
Boat is in the yard right now for blasting, isolation epoxy and bottom paint done correctly. Hey, its only money
Some good suggestions regarding the engine mounts. I have both the original design for outboard power and the inboard version which I prefer since it maximizes the all-important stern cockpit area with the full length swimstep. I'd like to stay with the inboard design, put the pods under the swim step and hang the engines off the edge of the swimstep. That would seem to give me the best of both worlds course it's a lot more complex than just notching the step for engines per original design. A limiting factor, though, is the engine height, the cav plate would be several inches above keel. I see most brackets allow the cav plate to raise a few inches, wish I knew what the limits were. Course the jack plates would be a possible way of dealing with this. Do you have any idea how high your cav plates are raised above hull bottom while you're on level plane?
Looks like you've got dinner hanging off your hull there! Guess you're going with an epoxy barrier coat then bottom paint? I saw an article on recent new products for alloy hulls in Natl. Fisherman, I'll try to dig it up and get it to you. Sounds pretty impressive. Thanks for the help.
What kind\\brand of bottom paint do you have on your boat? I have seen that type of corrosion from copper based anti-fouling paints applied directly over aluminum. Big no no. Or maybe thats what aluminum boats look like after being in saltwater. Man... freshwater is nice(except when its frozen )
Interlux reccomends their 2000 epoxy over bare aluminum then the antifouling of choice. Maybe by now there is a non commercial aluminum anti-fouling paint that is available.
Sim, there is no corrosion there, just looks like it. And no, the builder didn't use copper paint. He used the last bit of tin based paint I could find but he totally blew the prep & priming. I'm now going to the epoxy system base coat that provides a total galvanic isolation of the hull from whatever I put on the bottom. Now I can use anything, even copper.
Skypoke, I'm not entirely sure where my cav plates are in relation to the surface at cruise considering the setback. I haven't got the nuts to jump back there with the engines and look with the boat clipping along at 35 knots. I'm regularly adjusting them for different conditions so it varies. At rest with the jacks all the way up the cav plate is about 3" higher than the bottom of the hull and run down can be dropped to about 4" below. At speed on smooth water I run them pretty high. Prop styles have quite a bit to do with this as well and I've just taken off the 3 blades you see in the pics and have put on Merc 4 blade Offshores which allows higher settings. There are a lot of variables. Complicating things is the fact that my jacks change trim angle as they go up. This is OK but makes it tougher to evaluate the exact effect from raising the engine unless I trim in simultaneous to jacking up.
As far as how you build your boat it would seem there are several ways you could do it and none of them would be that tough to change later. With a skill saw in one hand and a mig gun in the other as you know it doesn't take long to move things around! I question how seriously you would opt to repower after going through the expense of putting on outboards, however. Are you going to trailer or leave it in the water? What is your beam? I've got a couple of ideas . . .
Alloy cat build update 
