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Questions about the Marissa


pbb

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I have a few questions about the Marissa design which don't seem to be answered by the existing posts or its web page.

 

 

1. Is it possible to say what is the lowest h.p. that would carry 6 adults at 25 knots in smooth water without struggling?

 


2. How can I can get a feel for what seating arrangements are possible for up to six adults? 

 

The photo shows a bench seat behind the centre console, but is there room for a second bench behind that?

 

Also, what options would there be for seating in front of the console? Is another bench seat possible? Is there scope for any seating going along the sides of the boat?

 

And also, what storage capacity is available at the front of the the boat. Is there space designed in for anchor, warp, boathook etc.? Presumably, there is accessible storage space inside the console as well?
 

 

3. Is there provision for any kind of swimming platform and boarding ladder at the stern?
 

 

4. Since I live in France, it would be sensible for me to order a kit of parts in Europe. Would it be possible to let a CNC shop over here cut the parts for this design, such as Jordan Boats (jordanboats.co.uk) in Scotland?
 

 

5. I notice that at least one builder had to experiment to get optimum propeller dimensions. Is that normal with a power boat design? Or are there build variables which affect propeller choice and which mean it can't be specified in advance?

 

 

Thank you




 

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Is it possible to say what is the lowest h.p. that would carry 6 adults at 25 knots in smooth water without struggling?

Firstly, let me say that I don't think the Marissa is really designed for 6 adults...according the the B and B website the weight of the boat is 520 kg and displacement at the DWL is 764kg leaving a payload of 244 kg. 6 adults with a little gear is likely 500 kg or more.

 

Having said that, looking at the graph below taken from Dave Gerr's "the Nature of Boats" you can see that 25 knots in speed requires a pounds to horsepower ratio of 40.

Assume 1000kg for boat + 6 adults, converting to pounds is 2200 lbs. HP required then is 2200/40 which equals 55 HP. As you say you don't want the outboard to struggle so you'll need about 75 HP running at 70% throttle.

Of course the bigger motor weighs more and would need to be factored in, requiring even more power.

post-425-0-63937700-1363225087_thumb.jpg

Peter HK

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Thank you for a very clear explanation, Peter - disappointing, but clear.

 

A payload of 244 kg seems to suggest that the design is aimed at a total crew of three adults or two adults plus two children - is that right?

 

From what you are saying I need to be looking at a boat with a much bigger difference between the boat weight and the displacement at DWL.

 

 

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A payload of 244 kg seems to suggest that the design is aimed at a total crew of three adults or two adults plus two children - is that right?

 

From what you are saying I need to be looking at a boat with a much bigger difference between the boat weight and the displacement at DWL

I think that's right, if you want the boat to perform in all reasonable conditions. Obviously you can load a boat up a bit more than the designed displacement in the right conditions but 6 adults regularly would be better served with a boat with a larger displacement.

 

Peter HK

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The Wooden Boat Magazine Eco challenge which the Marissa won asked for a boat that burned less than 2 gallons per hour while cruising at 15 knots with 650 pounds aboard = 4 adults. I think a crew of 4 is reasonable, but 6 would be a squeeze on a craft this size.

I mentally compare the Marissa with my 18 1/2, V hull aluminum runabout - that boat has a 1987 issued capacity plate for 8 passengers. Apparently folks weren't as porky in 1987. 8 people on board would mean they'd sort be laying in a pile since room is limited, so one must consider the comfort factor, and 6 on a boat that size is still kind of tight.

An Okracoke 20 might be a better choice for your planned adventures.

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Thank you for your advice. Peter HK and hokeyhydro.. However, having looked now at the Okracoke 20 it has the following figures: 

 

BOAT WEIGHT including 90 hp motor...1400-1600# (635-735 kilos) depending on materials and options*

DISPLACEMENT 2000 lbs (909 kilos) @ dwl

 

So if we say an actual weight of 700kg, that still only gives 200Kg to achieve the dwl. So doesn't that suggest it too would only handle 4 people?

 

The Okracoke 24 appears even worse with an actual weight of 1100Kg but a dwl weight also of 1100kg - surely that can't be right?

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I am still working with Yamaha to find a prop that will get the maximum for my 40 hp Yamaha. The current prop is a 11 5/8 x 11 can get no more than 5600 rpm. which equals about 25 knots. Yamaha is planning to send a rep. to help solve this and another issue of vibration at idle speed. I'll keep you posted. As to you question regarding seating- I think four adults is the maximum for any comfort on this boat, there is very little room behind the bench seat on my boat for another seat, room enough for moving around comfortably but not enough for another seat. The bow area where the "casting platform/anchor locker" is a good seating spot, there is plenty of space for anchor line forward of that.

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I think you are reading too much from the data.  While Marissa and Ocracoke 20 have displacements listed for the DWL, that does to mean that displacement is limited to that figure.  The DWL is likely Graham's intent as the design number for his calculations.  For instance, I was aboard the Ocracoke 20 with Eric and three other adults when it was making 42 mph as shown on the website.  The boat handled well at rest and in maneuvers at speed and the weight was no problem even though it was above the "design" number.  I would expect Marissa to act as well but I think 6 adults is more than it should be expected to carry and still meet the design goals.  The boat would still be safe if handled properly though.  Good judgement and seamanship should always be the final determinant of what a boat should do.

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