GT27 Build (Wes K)

[TomW1]

First to fallguy hump is so different on each boat that no calculator can predict what will happen in real life. I would expect after having done so many different boats that this boat one would be around 15 - 16, but don't quote me on that. Jacques discusses it in some of his designs, but not in this one. I agree I would not want to go above the hump but would like the HP to fight the winds. It depends on the design of the bottom panels.

Now Wes, propellor selection is based on the gear ratio of the motor. To small of a prop and the motor over revs, too large a prop the motor lugs down. Both conditions will shorten the life of the motor. I have given you the best choice for each motor to keep it at the max safe high rpm range and the roughly best fuel economy range based on the weight of the boat. You can always slow down if you want and check fuel usage and find your sweet spot. You saw what the prop size of the 50HT's were going to a 14" would be a very bad mistake and shorten the life of your motor and not provide you with any advantage. You will need a MPG gauge and run a course upwind and downwind of a mile or two starting at 1500 rpm's and going to 4500 rpm's to find your sweet spot. Anything over 4500 will just increase fuel usage.

My thoughts, the 50's or even better the 60's with all your windage would be my choice. For best fuel economy you won't be able to cruise at 8mph but 10-13 is possible. Some motors also have a sweet spot around 2500rpm's. I would also use a 4 blade propellor as it helps in tracking and in rough water is better than a 3-blade prop in holding onto the water and give you more control. Be aware some of the guys are reporting motor times of 6 months out from time of ordering. Once you are close to finishing and you can get some real weights, I will run everything through the calculator to get a real speed and prop number for you. Just let me know and I will let you know everything I need to be accurate. To put a 14' diameter prop from the calculations on your boat would require a 4" pitch prop which are not available, I checked, the lowest I found was 7". I tried for you.

I took a look at propellers with different geometries for HT motors and Yamaha has several available for barge/houseboats. Also the same for PT Props and Suzuki.

Boy, I have been windy Wes, please ask questions if you have any.

Regards. Tom

[wkisting]

Tom, thanks so much for your time and patient explanations. This is all new to me, as until now, I've been mostly a paddler/sailor and have little experience with powerboats (aside from a jet boat we had when I was a kid).

What you say about prop size makes sense. I'm feeling confirmed in my gut feeling that a 50 or 60 high thrust is the way to go. I guess I'd prefer the 60 for the same weight, but I'll have to go with what I can get. Suzuki seems my best option. And you're spot on... initial calls to area dealers suggested that it's a 6- to 7-month wait for a new motor if I order today. Ugh...

I'm hoping to get the side windows installed on our cabin in the next few weeks, and then I'll probably head for a scale to see if I can grab a current weight before I build out the interior. My guess is we're in the ballpark of 3,000 lbs. right now. If it's much more than that, I'll have to change plans on interior furnishings... My raw materials spreadsheet shows a total of 5384 lbs., but that is a mathematical calculation that includes virtually EVERYTHING, including weight of water, fuel, windows, paint, epoxy, furnishings, a rough-estimated 230 lbs. for a motor, and ALL raw materials (i.e., all fiberglass and wood supplies before all the cutoffs, sanding, etc. plus several sheets of plywood that are still on the rack, not yet used on the boat). So I think that weight total would have to be at least 30% higher than current actual weight, and hopefully more--but only a scale will tell.

One question: Can a boat of this size really cruise at 10-13mph with a 60 hp outboard? I thought somewhere Jacques had suggested the displacement speed limit was around 8, and that it would take 115hp to get up on a plane, which we aren't trying to do.

[fallguy1000]

Great question!

I think those numbers of 10-13mph are gonna be just under the hump. So the boat will be at maximum plow there and not cruising well.

Horribly inefficient, the boat will be running in the hole or in an out of the hole which is expensive on gas and not much fun.

Could I be wrong? Yuppers.

But I have a gut hunch not.

Planing is almost certain to occur at speed/rt(lwl) = 3

For you rt(lwl) = 5.2. So, you boat will plane around 15.6kts, call it 17mph, for sure. edit(if not super super heavy)

Running at hull speed is when the ratio is 1, or 5.2knots, call it 6mph

The space between is less certain. Between about 7mph and 16mph; you will be plowing water. The average of these two speed is roughly 11.5 or about 10 knots. That speed is fairly surely going to be bad and maximum plowing between 7-11.5mph, jist based on the s/l ratio.

Trim tabs can help push the bow down and get a big baby like this one out of the hole, btw.

[TomW1]

Wes your spread sheet seems to contain everything for the boat herself. Now, we need another one for the add ons, the people weightaand, coolers, ice, fishing gear, or people play gear, and anything else you might carry on as you get ready to are ready to take a trip. We want to bring the boat weight up to her weight as she leaves the dock.

I would finish building her out the way you want and then take her to the scale. No use shorting yourself and not haaving the boat you want.

For now, though give me your spread sheet weight of the boat and my spread sheet weight. I will then run it for the Suzuki 60AV and the Yamaha HT if you want. We will do it proper when you finish, again there is really no reason to take her to the scale until you finish her.

Tom

[wkisting]

Well, I would feel better knowing the current weight nonetheless. My spreadsheet is too unreliable because at this point it's nearly impossible to quantify how much of that weight made it onto the boat and how much was left to scraps, debris, and waste.

In people weight, I'm thinking worst case up to 6 adults at an average body weight of 180 lbs. (1080 lbs.) plus maybe 200 lbs. of carry on items (coolers get heavy quickly) . Most of the time it will be less as there are five of us, and my wife and three daughters weigh much less than 180 lbs. each.

With six hypothetical full size adults though, that doesn't leave me a lot of room for the weight of fuel, furnishings, water, and motor if I'm aiming for 5500 lbs. total... I'd love to know if my guess at the current weight being around 3000 lbs. is even close. Eventually, the scuppers become holes that sink us if overloaded too far....

[fallguy1000]

That boat is gonna be a lot of fun. Don't worry about the weight so much. But be prepared to move weight around to beach and exit the beach. I can tell you I've filled coolers a couple times on the bow in the middle of the nite cuz the wind changed and we were banging..

[TomW1]

Well, I would feel better knowing the current weight nonetheless. My spreadsheet is too unreliable because at this point it's nearly impossible to quantify how much of that weight made it onto the boat and how much was left to scraps, debris, and waste.

In people weight, I'm thinking worst case up to 6 adults at an average body weight of 180 lbs. (1080 lbs.) plus maybe 200 lbs. of carry on items (coolers get heavy quickly) . Most of the time it will be less as there are five of us, and my wife and three daughters weigh much less than 180 lbs. each.

With six hypothetical full size adults though, that doesn't leave me a lot of room for the weight of fuel, furnishings, water, and motor if I'm aiming for 5500 lbs. total... I'd love to know if my guess at the current weight being around 3000 lbs. is even close. Eventually, the scuppers become holes that sink us if overloaded too far....

Okay go ahead and get it weighed now. When you finish it, weigh it with it with tanks full and then do my spread sheet for people and every thing that you add on board. It can add up to several hundred pounds.

We want to get as close as possible to the weight as the boat leaves the dock to run through these calculators.

Regards, Tom

[wkisting]

My neighbor has me thinking about another possibility: twin 15 - 25 hp outboards. I would love the redundancy in case one motor ever went out.

Twin Suzuki 20 hp motors would come in at about 240 lbs. and even though they are not high thrust models, two 10-3/4" props would surely generate a lot more thrust/grip at low speeds than a single 50 or 60hp with a12" prop, no? Plus better directional control at very slow speeds. This is also about the same cost as a single 60hp.

Twin T25hp Yamaha motors would come in at 284 lbs. and are high thrust rated with a beefier lower unit for a larger prop. This is about 20 lbs. heavier than a single 60hp, but I bet it would offer superb control. This would add about $2K cost over a single 60hp, so that's one big downside.

Sounds like most of the downsides of twin engines are at the top end speeds when added drag can be a problem. But that's not really applicable to our case.

Of course this is only doable if both engines could fit within the 29-3/4" max mounting span of our engine bracket... not sure and I couldn't find definite numbers on min. mounting separation.

Thoughts?

[fallguy1000]

Modern engines are so reliable. I have a 2004 Merc 50hp. Hate to jinx myself, but the thing has like 2000 hours. Only problem ever had was voltage regulator and that was a design issue Merc had a campaign on that I never got a notice as 2nd owner. The VR failure kept knocking out my batteries, from undercharging.

For your boat, reliability is all about the batteries. A backup starter battery and/or dedicated battery protection device or both.

The house should be separate from the starts, and the engine on a houseboat always needs to be able to charge the house.

Victron makes some great stuff. And Bluesea..things like shunts and monitors. Some of it is really helpful. As a renter of houseboats, we always called into HQ. You won't have that privilege.

If you want advice about it; I'll offer what I can.

One thing about going too small on the engines is the alternators will be crap. Pay attention to this bit.

[Dougster]

I've only skippered a houseboat once. It was a big rental one with two outboards and you could turn it on a dime by reversing one engine and going forward on the other. Sure was easy to handle for me as a beginner.

Dougster