Increasing seaworthiness for a 7 m. motorboat

Discussion in 'Stability' started by chileflora, Nov 30, 2018.

  1. chileflora
    Joined: Nov 2018
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    Location: Chile

    chileflora Junior Member

    I recently purchased a motorboat hull in very good shape which I plan to adapt for coastal navigation. It is a model which is used in southern Chile in semi-sheltered waters (fjiords and Chiloe), but I am looking for something that would be capable of 4 m. sea / 30 knots.

    The size of this boat is 6.85 meters LOA, Beam 1.75 meters (real beam, without hull-deck joint, overall beam is 2.05 meters).

    I have two problems with this boat: I do not like very much the shape of the bow, the deadrise seems not ideal either, to say the least.

    But the main worry for me is the very narrow beam, and I am thinking of installing two floats (basically, hull extensions which would run from aft for about 5 meters, extending roughly 45 cm. to each side, for a real beam of 2.65 m, made from fiberglass and epoxy over plywood ribs, or plywood fiberglassed over plywood ribs. The cross-section of the floats would be 55 cm. high x 45 cm, semiround, and they would run along the vertical part of the board (topsides); I estimate their weight to be around 100 kg. In addition to increasing stability, it would also serve as positive flotation; the aprox. volume would be 2 x 750 liters. I am also thinking of putting two gasoline tanks of 110 l (long tubes with a 30 cm. diameter) in these floats...

    I would appreciate any input as to this idea and whether this configuration would indeed be safe for a reasonable sea.
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    It is a cathedral hull, and the type is not the best for offshore use, being inclined to a fairly harsh ride. Safety might be enhanced by adding a buoyancy band along both sides, but you might need to be careful with how that is done, or make for a worse ride. That style of hull will never be suitable for 30 knots offshore, it is more like a 15-20 knot cruise boat, as for 4 metres seas, well if it is a long swell, it is less of an issue, but otherwise it is better not to be out in a small boat in rough seas.
     
  3. chileflora
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    chileflora Junior Member

    30 knots = wind speed, not boat speed.

    I do not need fast boat, simply I want a faster boat than Catalina 22 (=5 knots). I have been cruising coastal pacific in Chile, and the sea condition limits I need are about 4-5 meters (let us say 4 meter significant wave height and 30 knot wind). With my modified Catalina this was about the limit. I got bored with the slow speed of the Catalina, and that is the main reason I am changing to a motorboat.

    What I am looking for to achieve with this boat is a speed of about 15-18 knots (economical plane) in good weather and 4-6 knots (no plane) in relatively heavy sea. But it must be safe...

    I will modify it to be a cruiser for two persons; the main problem is the range and I plan to have about 500-600 liters of fuel on board. Total displacement when loaded will be around 2400 kg. I have one 50 HP motor on another small motorboat which I plan to install in tandem with another 50 HP motor to be purchased.

    I bought the hull just a few days ago as it was half-priced. It was either take it or leave deal.

    My MAIN QUESTION is if installation of lateral floats will significantly increase safety for heavier seas. Comfort ride and performance (except for fuel economy) in good weather is not that terribly important.
     
    Last edited: Nov 30, 2018
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    30 knots wind speed is "stay ashore" conditions, unless you are a masochist, sustained 15 knots wind speed over a decent fetch of open water is about the limit of comfort in smaller craft. I very much doubt if 2 x 50 hp will easily hold on plane that boat at the weights you mention, though the cathedral hull form is a good weight carrier. You may find you are overworking the engines and using a lot of fuel in the process. But to answer your main question, yes you can improve the overall stability with some kind of buoyancy bands along each side, it night be enough to use a semi-circular inflatable tube, or slabs of polyethylene foam, but it you are going to put then too low down, or they present a flattish surface to the water underway, you will change the dynamics of the boat, and firm the ride up undesirably. This is the kind of thing I was mentioning with the foam, they go up to 7 meters boat size with these things, but they are a long way away from you !

    Kapten Boat Collars - the best stability & performance-aid for small boats http://boatcollar.com.au/
     
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  5. chileflora
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    chileflora Junior Member

    30 knots "stay ashore", but if caught in it, I need a boat "to stay alive". Usually 20 knot forecast is the limit I plan to go off, but sometimes these forecasts can get way off. So... I need a boat capable of 30 knot wind, not that I would routinely go into these conditions.

    You are possibly right about 2 x 50; My problem is that I have purchased a brand-new motor one year ago exactly with the idea of purchasing another and install them in a motor-cruiser (not defined up to this point). I never thought about the option of buying boat wtih used motors - I trust only new motors. I have 5 HP on my Catalina and it worked in appauling conditions for more than 400 hours, and still runs well.

    I do not even know the weight of the hull I have, I guess it should be around 900 kg. I think that if the boat will barly get on plane at 2300-2400 kg that would be great. One option is to run in displacement mode for the first part of the trip and then accelerate. I plan to run tests with one motor provisionally installed to see how it behaves in terms of weight/speed.
     
  6. chileflora
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    chileflora Junior Member

    This is the aproximate shape of the float I was thinking about. It would run along the topside (basically, it will cover exactly the long board which gets pointed towards the bow. The total length would be 5.2-5.3 meters.

    I was thinking of using something like 9 mm. or 12 mm. plywood ribs every 50 cm., glued in with epoxy and fiberglass.

    Then two options:
    - Cover it with fiberglass and epoxy; if one assumes that the floats should have the same thickness as a regular topsides, then based on Dave Gerr the thickness should be around 4 mm). The problem is the price, it will be about 60 - 70 kg, of which about 35 kg for resin = not very cheap. Also not sure how to prevent sagging of the fiberglass cloth, probably putting styrofoam and shape it. (polyrethane is difficult to obtain here, unless it is the plumbers type which is not verry rigid).
    - The second option is to cover it with 6 mm. plywood and throw may be 1.5 mm of fiberglass with epoxy.
     

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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You will likely get away with the 50's if you prop down (reduced pitch). What brand of engine ?
     
  8. chileflora
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    chileflora Junior Member

    Tohatsu two-stroke, no starter, no hydralic, no frills; it is relataively cheap (5000 USD)
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Your boat has the rectangular plan which helps make for stability, if you wanted to add a little something like the boat collars, it would not hurt, but anything more would probably be unnecessary, and start to look like a monstrosity, although it would also double as reserve buoyancy and insurance in what I assume are cold waters. Make sure the structure of the boat as it exists, is sound, before thinking about adding things on.
     
  10. chileflora
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    chileflora Junior Member

    The condition of the boat is great. And it costs only 2500 USD, which is very cheap by Chilean standards. I also like very much that it is single skin without fillers which tend to rot and much more difficult to repair or to modify.

    Waters are indeed cold, on Catalina I always had an immersion suit just in case in board, and staying afloat is a very important priority. The boat has sealed air pockets under the deck (V shaped spaces about 20-25 cm. deep), but they would barely provide positive flotation and almost guaranteed to turn the boat over in case of serious flooding. That is why the collar is so important to me. Turtling over is the worst scenario, both in case of hull damage and in big waves.

    Among some other things I plan to do is to increase the cabin size (extend it backwards by 50 cm. - room for bathroom with shower, the thing I most missed on my Catalina), make self-bailing cockpit and reducing the floodable cockpit volume (raising floor level by 15 cm, and making closed storage compartments), and raising gunwale by 20 cm. Below the raised floor in the cockpit I plan to install main gas tank.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I would be wary of raising the cockpit floor, that will raise the COG. The boat really isn't beamy enough for that, I would say. You are also talking about a very large fuel capacity, 7 metre power cats here would rarely have more than 400-450 litres fuel capacity, and these are wider and bigger boats, and use rather bigger engines than twin 50's.
     
  12. fallguy
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    fallguy Senior Member

    Well, I must say, this is interesting.

    What is seaworthiness is my question or point.

    First of all, the primary objective would be a vessel that would not sink. This requires buoyancy chambers or foam. If your vessel can sink; you have little business venturing out at a 20knot maximum.

    Second of all, your vessel must not drown engines. This is not the same as not sinking. Any extended submerging of engines and you will be disabled at sea no matter how many engines are back there. Avoiding drowning engines can be done by using scuppers in the right boat and they must be done well enough so they don't become a point of failure.

    Third, a disabled vessel must have a way to call for help.

    Fourth or further down the line is comfort. In a vessel this size; the last thing I'd do is raise the center of gravity by adding cabin or raising the cockpit(unless this affords scuppers).

    Fifth. Your 20 knot standard shows a lack of good judgement. I hesitate to suggest it because the other four are more important. But since you have this crazy idea amas are going to help; you might listen. Reduce your wind speed maximum to 14 knots as well, or more if that speed drives waves much larger than 6 footers. Slow swells are not what I mean either.

    Finally, a lot of people think a bigger boat means they can go out in bigger seas. This is only partial truth. If you wouldn't go out in a 5m boat; you have little going on upstairs if you think the 8m will work. Of course, there are a few exceptions and someone might jump on me, but get away from the idea that boat is safer than a 5m similar vessel and you might live longer.

    I would avoid the amas and I have some on a canoe. They can be built much lighter than the 100kg. If you insisted on something; a big foam n glass sponson might help, but the other ideas I offered are much better, or a different hull.
     
  13. chileflora
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    chileflora Junior Member

    To be frank, I never had a motorboat on sea, so this is my main worry - lack of experience with motorboats.

    However, I have sailed Catalina 22 in these areas (Southern Pacific fjiords and open coastal Pacific) where you are not supposed to take a 22 foot Catalina anyway. I felt very safe with my Catalina, but I made many modifications to it (www.sailingchile.cl), including heavier keel, reinforcement of keel box, reduced companionway, positive flotation, reinforcement of all attachment points for stays and shroud. Going out in 15-20 knots is more or less normal, and rarely you would expect to have less wind. I usualy awoided sailing out with forecast over 20 knots. Also, I always ran under engine (sails and engine), and only when the wind became too strong then I would switch to sails only (when prop would come out and rev up.)

    My main problem with it Catalina is that it is slow (5 knots), takes a long time to setup (3-4 hours before sailing, 2 hours after) , difficult to launch (110-120 cm. depth), no standing room, no bathroom. The most I endured in it was a 24 day trip to Laguna San Rafael Glacier (300 nm one way).

    And an absolute requirement for me is that the boat would be trailerable. Marinas here are very expensive, about 6000 USD/year for a small boat. If I would not be constrained by the trailerability requirement I would have purchased a bigger sailboat (maybe 33-35 footer), with a hull speed of 8 knots. That is the reason why I decided to switch over to a motorboat.

    I would never think of amas and loose things like this. They would either break off or require very serious engineering. I am thinking about rigid, round, relatively narrow extensions (inflatables are also very difficult to attach securely for heavy sea).

    Raising cockpit floor would not really raise center of gravity, because I would install gas tanks underneath. The usual way to take a lot of gas in these boats woud be just to put standing barrels (200 l), which is much less safer and with higher CG.

    Extending the cabin aft will obviously increase somewhat the CG, but I expect that the total weight of this will be less than 20 kg. But some comfort is required, and to compensate - again this idea of floats.

    Standard required equipment in Chile is obviously the radio, and life raft. I also always have a SPOT.
     
  14. fallguy
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    fallguy Senior Member

    Put the floor up above the WL so you can scupper. Add flotation foam if possible. Leave the cabin alone; find a place for portapotty in the current setup.

    The Catalina 22 is altogether a different boat. It is more like a fishing bobber at sea. Only time to worry is sails knocking her over or if you fill her with water somehow.

    Avoid stiff winds in the fishing boat and add scuppers and foam and fuel below decks. Avoid overloading as well. The other thing is to simply realize the boats shortcomings. If she is narrow; no reason to test her on the beam, for example.

    Lower your expectations and avoid high winds. When you see the storm a coming; head in. It isn't a big enough boat to take chances in.
     

  15. fallguy
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    fallguy Senior Member

    If you insisted on building sponsons; you could build them from xps. They would mould easily to the side of the hull and you would use foam glue to laminate the pieces together. Four pieces would give you 8", for example, perhaps 8' long. You would then form them by saws and hand. Xps is horrible at holding glass unless you sand it real rough. Others have scoffed at me here for saying it, but I couldn't even demould my xps part yesterday without delam from pulling peelply (yuk). Anyhow, sand it all real rough with 36 grit paper; hotcoat with thickened resin and lay it over with soemthing like 600g glass. It won't take impacts, though. If you want it to take some impact; you could add a wooden rubrail. The wooden rubrail could also double as the means to fasten them to the hull.
     
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