Amateur Q - Stability for small launch PV/EV retrofit

Discussion in 'Stability' started by RandyB, Apr 6, 2014.

  1. RandyB
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    RandyB New Member

    I am looking to do an electric retrofit to a small (21') harbor launch. We are looking to swap out the inboard diesel engine for electric, add batteries and an awning for solar. The question: Need to determine how the changes to weight (removing engine, adding batteries, awning, etc.) and windage (awning) will change the overall stability of the vessel.

    Given that the boat is currently floating, is there an easy way (inclination?) to test the current stability of the vessel, and then make some (rough) calculations, base on the weight changes to determine the final stability? and what would an acceptable change to the stability be? How to account for the windage of the new awning? I think my concern is, given the possible changes to stability, will there be an issue driving this thing at certain (hopefully very high) wind speed? or I guess the question would be more, at what wind speed does it become an issue?

    This thing will probably never leave the harbor, but obviously we want the thing to be safe and stable, regardless.

    Any preliminary guidance, advice on a general approach, things to think about, or some tools I should look at would be appreciated.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't know whether windage is going to be the main problem with the solar awning, it is after all not going to present much area to the breeze, even when the water is choppy, it is more likely an issue of the weight up there altering the COG. Maybe your battery set-up and electric motor can balance that well enough, it depends on the boat, have you any pictures ?
     
  3. RandyB
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    RandyB New Member

    Well, part of the project is to find the right boat at the right price... so at this point it's a rather theoretical, general question. We are looking at this one:
    http://scituate.mymarinepages.com/i...FT//21-FT/SCITUATE-MA-US/details-4458600#info
    Which I think is a good fit for what we are looking to do, and generally speaking whatever we end up going with will be similar.
    and yes, I was thinking that the weight of the awning could be an issue, raising the COG. Reducing weight of the awning and coming up with a streamlined awning system/rack is part of the design fun. I figured adding the batteries (depending on final size) would help reduce the COG. I'm just not real familiar, yet, with what kind of assumptions go into loading, etc. or what kind of stability is acceptable for a boat like this.

    There shouldn't be too much windage from the awning, that's right, it will be horizontal, but figured it would be worth asking about.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    That boat was probably a planing hull, if you want to go solar/battery/electric, that type of boat would not be advisable, it is designed to run at speeds that require power inputs well in excess of what current electic set-ups can provide. I just assumed you were looking at a 5-6 knot boat, and a planing hull would use more power even at that speed, than you might be able to easily muster from your set-up. It might not be so easy to find a hull designed specifically for low speeds, which is what you require, on the second-hand market. Trying to adapt a planing hull is not the way to go, imo.
     
  5. Rastapop
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    Rastapop Naval Architect

    Have you had a look at the weight change this will require?

    Batteries are very heavy; depending on the range you want you could be talking a huge increase in weight, even when replacing the ic motor with electric.
    How many kW (hp) are you looking at? How long do you want to be able to motor on a single charge?

    I'd guess issues from the canopy will be insignificant relative to the problems from other changes.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    It depends on how much running around the harbour will be happening. Presumably it won't be like going for a 100-mile cruise !
     
  7. Rastapop
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    Rastapop Naval Architect

    Yeah I just wonder if the OP has these numbers, and knows what the weight increase is likely to be ;)
     
  8. RandyB
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    RandyB New Member

    Again, things are still a bit up in the air, (pending some budgetary decisions), but on the high end, we were considering this Torqeedo solution:
    http://www.torqeedo.com/us/electric...ators-and-green-boaters/technical-information
    Thinking about the 40HP
    Now that you mention the planing hull, that's a good point. we were going to try to target something a bit higher end if possible, but not sure if even the 80HP Electric would be sufficient to push this on to a plane. And at lower speed (which will probably be most of the time) the hull form would be less efficient, as you point out. Might be worth targeting something more reasonable in terms of HP and speed, save some money and find a displacement hull. And higher speeds with an awning probably isn't really a good idea.
     
  9. Eric Sponberg
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    Eric Sponberg Senior Member

    RandyB,

    Welcome to the forum. A lot depends on what you want to do with the boat--is this for private use, or are you going to run a concession of some sort, taking passengers for hire? If you are going to take passengers for hire, then you will have to follow USCG guidelines for commercial vessels, you will have to have the boat inspected, and you will at least have to do a simplified stability test under USCG oversight when the boat is ready to be inspected. If you are going to use this as a private vessel, then you don't have to be so rigorous in your stability checks, but I understand that you want to be safe nonetheless.

    Diesel engines are relatively heavy, electric motors are relatively light by comparison. But there is the issue with batteries, which, in turn, are heavy, and they may not necessarily go into the hull in the same location where the diesel engine was, so you have some trim issues that you have to deal with.

    It is possible to make some estimations, but you have to do some measuring and weighing in order to come up with some reasonable answers. Let's say you have already acquired a boat. First, while it is afloat, measure the freeboards, which is the height from the deck edge to the water surface. Take these measurements at the bow and at each corner of the stern. Mark where the waterline is on the hull with an ink marker at the stern quarters and at the stem so that you can see the boat where the waterline was when the boat is out of the water. You'll need this information later. Take the boat to a truck scale and weigh it, then take the boat off the trailer and go back to the scale and weigh the empty trailer. The difference between the two weights is the weight of the boat.

    Next, you can estimate how much the engine weighs by getting some information from catalogs for that engine. Be sure to include the weight of the transmission and the weights of the fluids in them (oil and transmission fluid). Alternatively you can remove the engine and its associated running gear, like the tanks, and weigh them separately. Note their locations in the boat--measure their positions. Subtract all these weights from the weight of the boat to find out how much lighter the boat will be. It is also possible to do a weight moment calculation to see what the effect of trim is going to be on the boat by taking that amount of weight out.

    Next, estimate the weight of the new gear going in--the weight of the electric motor and its batteries. Again, it is possible to do a weight moment calculation to see where the best locations are to put weight back in. A good place to put the batteries might be where the tanks were. Keeping track of the weights and their placement will tell you if the boat is going to be heavier or lighter, sinking deeper or rising out of the water, and whether the boat is going to trim properly. Adjust weights accordingly, by calculation ahead of time, so that the boat is near to its original configuration and flotation. You can double-check all this after you do the work by weighing the boat again as before, and by measuring the freeboards afloat as before.

    The weight of the canopy will also have an effect, and that should be possible to estimate from how it is designed by calculating the weights of all its pieces. The metal parts will of course probably constitute the lion's share of the weight. Certainly, putting the canopy on is going to adversely affect the stabiliy of the boat (make it less). It is possible to do heeling tests before and after the changes to measure how much the stability changes. But again, you have to have a procedure to follow, and repeat again in order to estimate what the change is going to be.

    It is very difficult to do all of this without taking the necessary measurements n the actual boat, because you do have to know how the boat floats to begin with.

    Also, if you are intending to carry passengers, then adding the canopy is going to have a profound effect on the result over and above just its weight. The USCG is going to look very hard at the windage created by the canopy, and this is not just the profile area of the top and its supporting stanchions. They will calculate the full windage of the hull and the profile area of the canopy as determined by the outline of the canopy. That is, the full height and length of the profile area defined by the canopy and its supports above the deck,and this is added to the area of the hull--this full profile area is used in the windage calculations when determining how many passengers you can carry. This happens during the USCG inspection and stability test. They measure the boat and the windage area and they use that to determine the windage against the boat for the waters that the boat will be running in (protected water, partially protected water, or open water). They also calculate what the weight effect of the desired number of passengers is as they move to one side of the vessel. You check both calculations--windage moment or passenger moment, and whichever calculation causes the boat to heel more, that will determine how many passengers you can carry. There is an equivalency for windage = passengers, but it is different for every boat, so you will determine that passenger number at the time of the test.

    It is possible to be more accurate with calculations on weight and stability if you have the lines of the boat ahead of time. You measure flotation and weight from the lines of the hull and where you measure the waterine is. You have to get the hull lines from the boat's designer. Generally, it is very hard to do this--sometimes hull lines simply do not exist. That means you are back to doing measurements and estimates as above. It is also possible to go through the USCG inspection procedure ahead of time--do the tests yourself before you do it in front of the USCG just to make sure you know what to expect at the time of the inspection. You can make any changes as necessary so that you are assured of passing. Knowing how many passengers you can carry ahead of time will help determine how commercially successful you will be, and how much to charge each passenger.

    Finally, as to going fast under electric power, Torqueedo seems to lead the way when it comes to electric motors and batteries, but their 80 HP version has huge heavy batteries, and they are very expensive. That's the option for planing performance. If you decide on a displacement hull, the options for powering by electric motor are greater, and motoring for longer times at slower speeds is a lot easier to achieve.

    If all of the above sounds a bit confusing and involved, well, it is, and you might consider hiring a naval architect who is familiar with the procedures for inspection and to advise you on what the boat is capable of and what powering options would be better than others. The calculations are not difficult, but they are not entirely straightforward either. You cannot tell easily ahead of time whether one boat or another is going to work for the engine/motor conversion, at least not without doing some proper study, measurements and calculations on the boats involved--there is no quick and dirty way to check.

    I hope that gives some insight into the issues involved.

    Eric
     
    Last edited: Apr 11, 2014
  10. DCockey
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    DCockey Senior Member

    No USCG stability rules apply if the boat carries six paying passengers or less as an "uninspected" vessel. Also USCG rules do not apply to all waterways, in particular many lakes which are not accessible from the sea. However for those waterways state regulations may apply.
     
  11. Rastapop
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    Rastapop Naval Architect

    Here's some numbers for you then, using this page: http://www.torqeedo.com/us/electric-outboards/power-26-104-intelligent-lithium-battery

    80 hp = 60 kW
    One 55lb battery gives 2.7 kWh so at 60 kW that's 2.7 minutes of discharge.

    Which is 20 lb of battery per minute of 60 kW motoring.

    And it gets worse: http://www.torqeedo.com/us/electric...lboats-up-to-4-tons/technical-data-dimensions

    If we say (generously) that the efficiency is 60% based on the numbers on that page, you're looking at over 30 lb per minute at 80 hp output.

    How many minutes do you want to motor before you recharge?
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Actually short intermittent hops around the harbour might be just what an electric boat could excel at, but something that demands 80 hp, I doubt would fit the bill.
     
  13. PAR
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    PAR Yacht Designer/Builder

    That boat looks to be the Mariner 20, the Fish and ski model (there's a cuddy cabin model too). It's a full plane mode hull and will do very poorly as a displacement speed craft. With a 25 HP outboard, it'll not get up on plane and will plow along with her bow in the air, in the 9 - 10 MPH range. With a 40 HP outboard you'll manage to get up on plane, but contrary currents and windage might slap you off every once and a while. Speeds will be in the high teens, maybe 20 MPH on glass smooth water. She's really designed for much more power. A 75 HP outboard would be what she wants, which will drive her to the high 20 MPH range. Her max HP recommendation is 200 HP (I/O), which should offer a clue as to what she really is.
     

  14. RandyB
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    RandyB New Member

    Eric's answer was exactly the kind of info I was looking for. Thank you very much for the explanation, Eric. That boat in the link was just a passing thought really. I think we are going to go with something like a classic cat boat, with nice displacement hull, nice lines. easy to push at low speeds. Just to cruise around the harbor with cocktails in hand at sunset and feel classy, and quiet. the larger Torqeedo and higher speeds will really not be necessary either, just wanted to 'float' some possibilities. cheers,
     
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