Im thinking about making a very scaled down version of my 28's hull to try out a propulsion idea I thought of. Money wont allow a full scale test so I thought I may try my hand at a scale model. If i can get a good master ill make a mold from it. I dont want to divulge my idea quite yet but wanted to run some things by yall.
What material do you think would be best to make a plug from? I dont have Randalls skills so if I attempted something in wood I hate to think how it may turn out. Like I said though i would like to make a mold from it so it would need to be durable enough for atleast one pull. I have used plaster of paris once and it worked pretty decent. The end result ill probably lay up in fiberglass.
Do any of you have any tips for pulling certain measurements from the hull or drawings to make it as close as possible? I will probably keep my math simple and make 1"= 1'. If that looks too small for power options ill change the scale to a little larger.
Would most of you agree the optimal placement for the "thrust" would be where the props are currently located? If you could move that where would you place it? The same goes with the rudders.
From a drag and efficency standpoint would you think a single rudder on each side would be more efficent than a dual shorter rudder on each side. Try not to consider the prop size in that since there wont be a conventional prop there. Another option could be shorter and longer from front to back as long as it woulnt hit the hull in a turn. A rudder too far back like the transom would make it handle more like outboards right? From what i've read on that and the 28 you just dont have as good of control.
I am doing this mainly out of curiosity and it may or may not be worth anything good. My ideas typically work better in my head but this one i want to try. Our 28 is still sitting high and dry unfortunatly so I thought if it worked on the model i could try it on that since its not repowered yet. I have always wanted a model of the 28 too.
Thanks for any help or ideas. Sorry for the suspense on how the power is getting to the water too.
Scale Bertram Hull
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In Memory of Vicroy
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CaptPatrick
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Yea I know what you mean about the jet drives. I've never drove anything with them but seeing people in the little jet boats makes them look a little like they are hard to control. It seems like the reverse thrust with them would almost be negated by the back of the boat too. They always seemed a little fragile with all the moving parts back behind the boat but then again a piece of rope below the surface probably wouldn't bother it compared to shafts. Guess thats one of the give and takes.
Vicroy: What ever happened to the Buck marine diesel guy that used to post here. That sure got quiet fast. I dont know enough about engines to get into that and cant afford anything that may claim to be "green". For some reason that and hybrid seem to be like racing stripes but instead of adding HP they add cost. :) I wish someone would come out with something good though.
My idea comes after the engine to put the power to use. Its a matter now of what the gains and losses are if it would even work.
Vicroy: What ever happened to the Buck marine diesel guy that used to post here. That sure got quiet fast. I dont know enough about engines to get into that and cant afford anything that may claim to be "green". For some reason that and hybrid seem to be like racing stripes but instead of adding HP they add cost. :) I wish someone would come out with something good though.
My idea comes after the engine to put the power to use. Its a matter now of what the gains and losses are if it would even work.
1977 B31 (315 Cummins) Build thread --->https://www.thehulltruth.com/boating-ho ... model.html
2010 Key West Bay Reef | 150 Yamaha
1986 Bertram 28 260 Mercruisers [SOLD]
2010 Key West Bay Reef | 150 Yamaha
1986 Bertram 28 260 Mercruisers [SOLD]
I think im going to end up making the plug from plaster after doing more reading. That seems like the best way for me to make the plug and seal it before glassing it to make the mold. Do you think i should make it more like 3' long to get a more realistic model to try things on? That will kill the sit around the house abilities but may make a better "tool".
I noticed that Albemarle used an outdrive on some of their boats. http://www.albemarleboats.com/trailerable/jps.html Whats the real reliability and useability story on these units. It looks like they may be the Volvo units but use a driveshaft instead of a more direct inboard/outboard setup. How would the handling be on something like this? Im just curious what kinds of downsides you would have. You would have the drag of an outboard, the weight dirtribution of an inboard (for the most part) and the possible fuel economy of your inboard of choice. Having never seen or used one the Volvo Duoprop deal is pretty interesting.
The 28's with outboards fly but arn't really useable going that fast in normal non glassy days. They also have the center of gravity shifted back and weigh less to help get up on plane. I dont want to start an outboard debate because im not going to go that route though.
I noticed that Albemarle used an outdrive on some of their boats. http://www.albemarleboats.com/trailerable/jps.html Whats the real reliability and useability story on these units. It looks like they may be the Volvo units but use a driveshaft instead of a more direct inboard/outboard setup. How would the handling be on something like this? Im just curious what kinds of downsides you would have. You would have the drag of an outboard, the weight dirtribution of an inboard (for the most part) and the possible fuel economy of your inboard of choice. Having never seen or used one the Volvo Duoprop deal is pretty interesting.
The 28's with outboards fly but arn't really useable going that fast in normal non glassy days. They also have the center of gravity shifted back and weigh less to help get up on plane. I dont want to start an outboard debate because im not going to go that route though.
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In Memory Walter K
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In my opinion, if you fish, you will find it in the way and a pain in the ass. Even if you don't, it's one more thing to go wrong. Volvo parts are notoriously expensive and are quite often not in stock. The plus side might be that duoprops track straight and you won't have rudders. Not for me, though.
Bertrams are of course known for their deep V hull form. This hull form is not very efficient when it comes to power consumption.
In order for a boat to plane out it has to press downward on the patch of water in which it is running with enough force to equal its own weight. If the bottom of the boat is quite flat in section then a lot of water is pushed downward, and relatively little outward. This creates the lifting force to keep the hull up on a plane fairly efficiently.
On the other extreme a deep skinny hull with slack bilges (think like a keel sailboat hull) mostly pushes the water sideways and pushes very little downward making it very difficult for it to get up on a plane.
In between we have our deep V hulls. They push the water downward, but due to the V shape of the hull they are also pushing it to the side at the same time. The stuff pushed to the side does nothing to lift the hull and represents wasted energy with regard to getting up and planning. It is only the water pushed downward that contributes to the lift.
Of course the deep V has a lot of other good things going for it that I don’t have to enumerate here that more than make up for its inefficiency when compared to a flat bottom hull.
The point I’m coming to is that the weight of the total package has a huge effect on performance in a deep V design. And in smaller boats the weight of the engines is a bigger part of the percentage of total weight overall. So it isn’t just the maximum horse power that matters for the power choice, it is also the weight of the power plant and its associated gearboxes mounts and drives. Then there is the fuel you would like to bunker; The big gassers need the most fuel to run a given distance, and that weighs the hull down more…. Outboards use less fuel (because of their lightness meaning they are putting out less HP to maintain a certain cruise speed) and so they can bunker less fuel and thus less weight and go faster still.
So power choices as they effect total weight are much more critical in our deep V’s than in a flat bottom boat design.
The fellows over on the 25 board have experimented with all sorts of different power options from heavy inboard Diesels to light single outboards and the general outcome is as follows:
Outboards are light and therefore don’t require much horse power to get up and go really fast…. But what you give up is some of the “wave crushing†ability of the deep V Bertram hull. At the other end there are the heavy inboard motor powered versions that use a lot of HP, don’t top out at the same impressive speeds as the OB’s, but shrug off chop and seas like nothing at all.
In between is everything else more or less along a spectrum that you can imagine.
If you are starting with a clean slate you first have to ask yourself what type of use do you want out of the boat. Offshore a lot? Going long distances? A big heavy efficient Diesel is the key. Always going to be inshore or nearshore and you want to go fast? Outboard is the hot choice… at least in the Bert 25 hull, but the principles would be similar for the 28.
Out drives of any sort are miserable maintenance problems. They could be better if someone took the engineering seriously, but they don’t. Not Merc, not Volvo. At one time I saw an outdrive at a boat show designed for strictly commercial and military service that intrigued me. Unfortunately the salesman was a bit of an ass and wasn’t interested in discussing a non-commercial application with me… I think they are out of business now. As a practical matter your choice is between Mercruiser or Volvo, both are a maintenance headache. The only good thing about an outdrive is being able to lift the drive for beaching the boat and for trailering.
In the 60’s when outboards were not as massive nor as reliable as they are now the I/O seemed like a nice marriage of inboard-engine torque and reliability with the convenience of a lifting drive. But now straight outboards come as big as you could want and they are reliable as well, so they are a better choice.
It is pretty straight forward to put a jack shaft to an outdrive with the motor mounted further forward. This has one advantage of moving the weight forward in the hull which for some designs is a must. This is especially true for hulls that were designed for inboard power originally. Putting a couple of heavy outboards out back on a bracket off the stern of such a hull (of which the Bertram is one) leads to having to add ballast forward to correct the CG. If you were to consider outboard power on a Bert you should seriously consider a notched transom for this reason and forget about the bracket approach.
Peter
In order for a boat to plane out it has to press downward on the patch of water in which it is running with enough force to equal its own weight. If the bottom of the boat is quite flat in section then a lot of water is pushed downward, and relatively little outward. This creates the lifting force to keep the hull up on a plane fairly efficiently.
On the other extreme a deep skinny hull with slack bilges (think like a keel sailboat hull) mostly pushes the water sideways and pushes very little downward making it very difficult for it to get up on a plane.
In between we have our deep V hulls. They push the water downward, but due to the V shape of the hull they are also pushing it to the side at the same time. The stuff pushed to the side does nothing to lift the hull and represents wasted energy with regard to getting up and planning. It is only the water pushed downward that contributes to the lift.
Of course the deep V has a lot of other good things going for it that I don’t have to enumerate here that more than make up for its inefficiency when compared to a flat bottom hull.
The point I’m coming to is that the weight of the total package has a huge effect on performance in a deep V design. And in smaller boats the weight of the engines is a bigger part of the percentage of total weight overall. So it isn’t just the maximum horse power that matters for the power choice, it is also the weight of the power plant and its associated gearboxes mounts and drives. Then there is the fuel you would like to bunker; The big gassers need the most fuel to run a given distance, and that weighs the hull down more…. Outboards use less fuel (because of their lightness meaning they are putting out less HP to maintain a certain cruise speed) and so they can bunker less fuel and thus less weight and go faster still.
So power choices as they effect total weight are much more critical in our deep V’s than in a flat bottom boat design.
The fellows over on the 25 board have experimented with all sorts of different power options from heavy inboard Diesels to light single outboards and the general outcome is as follows:
Outboards are light and therefore don’t require much horse power to get up and go really fast…. But what you give up is some of the “wave crushing†ability of the deep V Bertram hull. At the other end there are the heavy inboard motor powered versions that use a lot of HP, don’t top out at the same impressive speeds as the OB’s, but shrug off chop and seas like nothing at all.
In between is everything else more or less along a spectrum that you can imagine.
If you are starting with a clean slate you first have to ask yourself what type of use do you want out of the boat. Offshore a lot? Going long distances? A big heavy efficient Diesel is the key. Always going to be inshore or nearshore and you want to go fast? Outboard is the hot choice… at least in the Bert 25 hull, but the principles would be similar for the 28.
Out drives of any sort are miserable maintenance problems. They could be better if someone took the engineering seriously, but they don’t. Not Merc, not Volvo. At one time I saw an outdrive at a boat show designed for strictly commercial and military service that intrigued me. Unfortunately the salesman was a bit of an ass and wasn’t interested in discussing a non-commercial application with me… I think they are out of business now. As a practical matter your choice is between Mercruiser or Volvo, both are a maintenance headache. The only good thing about an outdrive is being able to lift the drive for beaching the boat and for trailering.
In the 60’s when outboards were not as massive nor as reliable as they are now the I/O seemed like a nice marriage of inboard-engine torque and reliability with the convenience of a lifting drive. But now straight outboards come as big as you could want and they are reliable as well, so they are a better choice.
It is pretty straight forward to put a jack shaft to an outdrive with the motor mounted further forward. This has one advantage of moving the weight forward in the hull which for some designs is a must. This is especially true for hulls that were designed for inboard power originally. Putting a couple of heavy outboards out back on a bracket off the stern of such a hull (of which the Bertram is one) leads to having to add ballast forward to correct the CG. If you were to consider outboard power on a Bert you should seriously consider a notched transom for this reason and forget about the bracket approach.
Peter
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