5.2m-5.5m Plate alloy all-rounder

[salty1]

Hi, I've been trolling the forum for quite some time, trying to get a clue on various hull designs.

Essentially, I'm building (or buying) a plate alloy family boat for fishing and skiing between 5.2 and 5.5mt. There are a billion existing options, which makes it difficult to determine fact from marketing spin.

• What is a good deadrise compromise for up to .5mt chop / 1mt swell? (or variable deadrise?)
  
• Do reverse chines work as proposed?
  
• Is a delta pad likely to generate real-world gains up to 35mph? (lower planing speed, fuel efficiency, higher top speed?)
  
• With or without keel?
  
• What is the benefit of strakeless designs? (Don't they lose lift)
  
• is there a golden rule for beam vs length? (tradeoff stability for wave piercing ability)
  
  or can you recommend any great hulls?
  
  Any help greatly appreciated.

[whitepointer23]

google barcrusher boats salty. you get the soft ride and stability. there heaps of them down here and the owners love them.

[salty1]

Thanks guys. I have looked at local manufacturers like tabs, barcrusher, fisher, bluewater, stabicraft etc. Unfortunately, none of them near here have any I can test drive, so I'm trying to gather info on what makes a good hull.

I've even considered specmar, as they will customise (strakes, chines, deadrise, delta pad, beam etc), however without hard data, I would probably design a monster.

[Kiwifinn]

Hello Salty,

My two bits worth of what makes a good planing hull, keeping in mind that I like to design for comfort and efficiency:

1. Long waterline for increased footprint which reduces bottom loading (more lift for given weight)
2. Fine entry to reduce vertical accelerations (slamming)
3. Minimise weight within reason for efficiency.
4. Moderate deadrise at transom again for efficiency. This also helps with stability at rest.

This type of hull is to be ridden with the hull in the water so the fine bow can get to work dissipating the wave impact, trimming the bow up will slam in waves if the deadrise aft is not very deep. Obviously the chine in the bow need carefull concideration to keep spray at bay.

I have not drawn any spray strakes to my designs, have not been required.
Reverse chines increase lift by keeping waterflow under the hull where it can generate lift. My preference is flat chines though in light weight long waterline hulls.

The alloy boats in the size range you are looking at tend to have raked stems which lead to short waterline lenghts and a rather blunt entry as a result. The reason for this is that many are built starting with the hull panels into which the internal components are fitted. To get a reasonable fine entry with a long waterline length the boat would need to be built like larger boats are built, ie. erecting the bulkheads and longitudinals onto a strongback around which the hull panels are bent. Bending the plate over a framework allows much better control of the shape and more curvature to be applied.

Some builders feel that this type of construction is not economical in the smaller sizes.

Am I correct in assuming that Barcrusher are known as Surtees here in NZ? They certainly look very similar.

[salty1]

Hi Kiwi,

Am I to assume you are referring somewhat to the yamaha/southwind longboat styles? I do like them, though I'm told they are a very wet boat in a quartering sea. Perhaps that could be remedied by spray deflectors???

I believe the "specmar/platemaster" boats are built around a backbone and internals before the plate. Though I suspect that flaring is difficult to achieve (to conform to a shape akin to dean's design)

Barcrusher and surtees look almost identical. Both are available in OZ.

[Kiwifinn]

Salty,

I had not seen the Yamaha long boat before, interesting boat! I can see they would be a bit wet with the low chine and sheerline forward.
Flare is not difficult to build into a hull but can be a bit tricky to design.
The boat in the picture is one of my designs showing the characteristics I mentioned before. She is 32' so a fair bit larger than what you are talking about here but shows that plenty of shape can be achieved even with fully developable shapes.

[Ilan Voyager]

Salty, you're asking for a basic open 18 feet FOR FISHING AND SKIING . So staying in the capabilities of the safe familial use of a motor boat of this size; A few (2 or 3) miles maxi from the shore, in the close range of an harbour if the weather goes bad, and sea state 2 or everybody will start to puke. It's just a 18 footer.

For skiing 100 HP is the bare minimum and you need speed and acceleration. That the main requisite. So deep vees are useless except if you like gas heavy drinkers to move it.


Kiwifinn is right:
Fine entry with 12 degrees stern, width around 1/3 of the length. A delta pad can be delicate for the lateral stability of such a small boat, no need of a keel, strakes are useful to keep the spray out.

I shall add that polyester boats on this size are lighter than alu and probably cheaper. Alu plating in small sizes has to be thin and the boat will be easily bumped.

I'll add that a simple design will work well enough, no need of complications. I would look more on 2 things; dry or wet? some boats can be very wet, not a fun for the family and the essential: security, unsinkable or not? stability? good safe seating? easy to get in from the water?