Playing around with a 10 m trailer cruiser

[marshmat]

It seems when I'm not designing things for school or for work, I somehow end up designing things just because....

I've been thinking lately about what to build when my Bolger-designed runabout, Sunset Chaser, now in her seventh season, starts to feel a bit too small. I'm looking at least five years down the road here, and all of this is of course hypothetical, but it's fun to dream.

The mission is an efficient, comfortable boat for use on inland lakes, rivers and canals, with inshore ocean cruising in fair weather being within reach. Extended cruising for two and long weekends for 4 should be comfortable, with the capability to host as many as 10 on day cruises. The ability to gunkhole around in log-strewn and rocky bays is essential, as is the ability to be towed cross-country on a standard licence, ideally with nothing larger than a 250-series pickup. A 25-knot cruise should be possible with a reasonable load, but the ability to maintain a smooth, level, stable and efficient attitude right through the 'hump speed' phase is more important.

I'm currently on the seventh loop around the design spiral with this one and I suspect it'll take at least another forty before I'm happy with it. (Version 6 is shown below.) But at this point I think the concept is ready for my friends on here to take a look at. I haven't done anything on the pilothouse, deck, interior, etc. yet, right now it's just hull concepts and a preliminary weight and balance estimate (~3500 kg in running trim, full tanks).

So what we have here is a shallow V low-speed planing hull, slightly soft chines at the transom smoothing to rounded chines as we move forward. The running surface is nearly monohedral at around 10 degrees for the first 3.5 metres forward of the transom, sharpening to a fine 30-40 degrees where the entry point would be when planing. No rocker or hook in the aft parts of the running surface, and I'm trying to keep the bow sections just a little bit convex. Construction would be in wood/epoxy strip plank. At present the concept is being drawn with a single waterjet drive in mind; it seems so far that hull resistance at 25 knots should be no more than 7-8 kilonewtons.

Suggestions, comments and recommendations of other vessels to take inspiration from are welcome (I've already taken inspiration from about 20 other vessels already mentioned on the forum, esp. in the "option one" project). Commentary on the probable running habits of the hull or experience with similar shapes is especially so. Keep in mind that this is just a concept at present and if it does ever take to the water, that's at least five years off. The linesplan below shows more or less what I have in mind for the hull, awaiting scrutiny and review. (Delftship/Freeship is being used for the preliminary sketching.) GIF format for now so everyone can see, if there's requests for different formats I'll post them.

 

[FAST FRED]

Looks good but don't boats of that style usually have some sort of chine to keep she spray at speed under control?

"A 25-knot cruise should be possible with a reasonable load, but the ability to maintain a smooth, level, stable and efficient attitude right through the 'hump speed' phase is more important.".

Estimated mpg?

FF

 

[marshmat]

Good point Fred.... perhaps a spray rail will be added. I very much like the ride and motion of a soft-chine hull though.
I haven't got around to calculating fuel consumption estimates just yet; I find it preferable at first to work towards minimizing hull resistance, and then look at powering. Right now I'm thinking of a lightweight marine diesel with a waterjet drive, but I'm not far enough along yet to know for sure.

 

[marshmat]

Round seven....
I figure a slightly flatter bottom with a bit more planing area wouldn't hurt, given the conditions this boat would see. The bottom's been widened a bit in the aft sections, with the chines being somewhat more square. Transom deadrise at the keel is still 10 degrees but that shallow angle is now maintained over a somewhat wider planing surface with a little bit more surface area. I figure this should improve planing lift in the aft sections and stiffen the at-rest motion a bit, at the expense of a slightly harsher ride in bad chop.
The spray rail's coming soon, just trying to think of how to get it to flow nicely with the hull lines.....
I'm thinking that one of the lighter 150-200 hp marine diesels spinning a Hamilton HJ 241 or 274 waterjet would be a good match. It looks so far that no more than 8 kN of thrust at 25 kts would be necessary but I haven't gone into too much detail on that yet. I know waterjets are usually best at higher speeds but I think if I were to build this one that I'd prefer the minimal draft and the absence of "sterndrive headache", at the expense of a bit of low-speed efficiency.

 

[Guillermo]

Nice project and nice lines, Matt.
To my taste I rather prefer #6 ones, just making the stern wider as to bring sides almost parallel all the way back to stern. I think for speeds in the range of 20-25 knots, the more warped half rear body lines of #6 look to me more adequate than the #7 ones, and the softer bilge radius will be easier to build in strip planking. Spray rails will also proof useful, in my opinion.
As per a very, very rough estimative, I think it will need close to 200 HP to get a 22 kn cruise speed for the stated 3500 kg disp with a jet, so perhaps a higher rated engine will be required....?

Cheers.

 

[marshmat]

Thanks for the comments, Guillermo; much appreciated. More drawings will certainly follow....
Good point about the sharper chine making it harder to do a strip-plank build. It would seem to me though that widening the stern much more would encourage it to stuff the bow down when in a following sea, an irritating trait of my current boat that I'd like to avoid.
I haven't actually drawn in the wheelhouse yet (working on a few different styles but aiming for sort of a Downeast lobster-boat shape) but preliminary estimates of the size and location of the remaining bits seem to put the fully loaded weight (full fuel/water, 4-6 crew, plenty of supplies) right around the 3500 kg design displacement. I've been told that for a low-speed planing hull the centre of gravity should be around 1/3 of LOA forward of the transom; this is about the balance this one has now. Does this sound about right?
As to power. At 24-25 knots, which is the highest speed I'd expect this boat to be able to sustain for any period of time, 8 kN of hull drag translates to approximately 100 kW of total losses. (That 8 kN is still just an estimate, I have yet to finish comparing the results of different resistance calculations.) Bottom loading on plane is around 200 kg/m2 fully laden. Let's say the inefficiencies in the jet are around 30% as the jet makers often claim, that translates to 145 kW or 190 hp at the driveshaft. Factor in a bit of gearbox and drivetrain loss and something like the 220 hp Yanmar 6BY220 or similar would appear to be sufficient (220 hp peak, 170 hp continuous, burns around 32 L/hour at 3600 rpm), yielding 63 hp/tonne. This seems to match well with HamiltonJet's average performance curves for a single-jet boat of this size and weight. Rough guess would thus be around 1.3 litres per nautical mile or about three nautical miles to the gallon at the 25-knot maximum cruising speed. A less aggressive cruise speed in the high teens would hopefully get closer to 5 nmpg, perhaps a bit better under light load. Sound reasonable?

 

[Guillermo]

I think it makes sense. Perhaps power would need to be a little bit higher, in my opinion, but maybe I'm wrong.
Cheers.

 

[marshmat]

9th round....

I think I'm closing in on something I like here

I've been playing around with a few planing resistance methods the last few days, especially a couple of the Savitsky-based spreadsheets posted on here (Dingo's is especially good, in my opinion). The results have convinced me she needs a bit more lift in the stern; I've brought the transom deadrise down to a hair under 8 degrees and widened the aft planing surface a touch. The midships sections are more or less unchanged while the entry is a tad sharper. The chines have been softened a bit and the sides are closer to vertical at the stern. Perhaps she'd look a bit classier with the transom raked back 5 deg. or so, I'll see how that looks in the next version.

It looks like my preliminary resistance estimates were somewhat high. Now that there's actually a hull to work from, it looks like drag in loaded condition will actually be closer to 5-6 kN at 12.5 m/s (25 kts), or about 65-75 kW of total hull losses at maximum cruise. It seems Savitsky's method isn't quite meant for soft-chine hulls but with some simplifying approximations to the geometry I figure it's close enough. And although this is probably wrong in every sense of the word, I also ran it through Michlet (with appropriate guesses for trim and -ve sinkage, aka lift.) That yielded drag estimates that, while somewhat different in shape, were on the whole roughly comparable. I'm thinking of trying a run in Fluent, but that program gives me enough of a headache in plain air, let alone a two-phase flow with a free surface.

Based on what I have now, it looks like a diesel peaking at no more than 200-220 hp would be more than sufficient for what I want; remember this isn't a fast boat, there's no need for it to do much more than 25 knots. She'll probably trim about one degree down by the bow at rest, and likely never more than 4 deg up throughout the transition speed range.

The hullform will probably continue to evolve little by little; I'm going to start working on the structure and interior in more detail soon. I'm thinking of putting a model together to pull beside the current boat for some tests, perhaps that will shed some more light on it.

 

[Guillermo]

Nice forms, Matt. I like them.
Savitsky should be good enough at this stage.....
Cheers.

 

[lazeyjack]

Matt, my comment, this boat will be wet, the spray will lift up the topsides in a plume and if the wind blows, will soak the boat,

I like this sort of boat they always run quiet,
I could help more with the trailer, I had a die cut, 130x75x8 6061T6 ally. I used to make these for all my trailer boats to 4 tonnes
the beams were bolted in naturally because you never weld across a girdar(chassis rail) never had a failure in all those years For larger boats used air over hydraulics, where air was avilable, and override sometimes, on boats to 1.5 tonnes

 

[marshmat]

Just a few more quick-n-dirty pics of it here, version 10....
I finally found a line I think I like for the spray rail (it protrudes a good 2" from the hull, more than double the size of the fairly effective rail on my current, smaller boat). A parabolic-entry nacelle pod for the jet intake has been added (she doesn't have enough draft for a pump to prime well in a conventional keel-cutaway installation). All the drives being considered transmit their thrust through the intake plate so there's gotta be room in the nacelle pod for some beefy stringer tie-ins as well. Superstructure is in work and will be posted when I'm satisfied it isn't completely butt-ugly

 

[Guillermo]

You have to take into account the jet intake is placed at the more charged area when planning, and that diminishes the lifting force. That will go against performance, increasing the power needed.
I think the position of the spray rail is somewhat high.
Cheers.

 

[marshmat]

You have to take into account the jet intake is placed at the more charged area when planning, and that diminishes the lifting force.

Good point.... I'm trying to figure out just how big this effect will be, none of the jet makers seem to offer any numbers on the loss of lift due to the intake. Suggestions?
Most pumps that would be suitable have intakes from 80x40 to 100x45 cm, or a bit less than 0.5 m^2 out of a hull with a planing area of 13 to 16 m^2 (depending on speed/load). That's around 3-4% of the lifting surface that's lost to the intake, albeit right at the transom. Seeing as the water in that area is actually being diverted upwards into the boat, it would seem the actual reduction in lift has to be slightly higher than 3-4%, but by how much? (I'm figuring the jet nozzle would angle down by the standard 5 deg, ie. normal to the transom, which should partly offset the loss of lift from the intake.....)

Some might be wondering why I'm so insistent on jet drive for this design, given that jets are usually thought to be inferior to propellers below around 20-25 knots. There are a few reasons. First is that if I were ever to build this one, I want to be able to run with 8-10 crew and full tanks, as well as being able to run with 2 crew and no gear, and anything in between. There's more than a 30% weight difference between loaded and empty cases and the efficiency of a jet does not depend on weight nearly so much as a prop does, nor can a jet overload the diesel if the boat's too heavy. I'm fairly convinced from the data and spec sheets I've found that it is indeed possible to select and tune a waterjet to come close to prop efficiency in the speed range I'm considering, if the focus is shifted away from top-end and in favour of balanced mid-range performance. Also, I'm sick of outboards and their many ills, as well as sterndrives and their maintenance headaches- a jet seems much simpler. Finally, I like to know that I'm not going to be crippled mid-lake if I run over a piece of submerged driftwood.

 

[Guillermo]

I'd say around a 5%, but I'd consult with OTTO RANCHI, who has a good deal of experience with jets. PM him.

 

[Willallison]

Matt - I reckon that almost 1/2 the people on these boards are or have at one stage, played with something very similar. I've been at it on and off for a few years!

I'd concur that this has the potential to be a wet boat. Also that your spray rail is too high. It really needs to be at the turn of the bilge to be really effective. I'd add at least one more lower down too.
I disagree that your looking at running trim no more than 4 degrees. The soft chine will add about 1 degree and the 66% lwl location of your cb/cg is too far aft, so will probably add another degree at lower speeds. Add this to the loss of lift around the jet intake and I reckon your looking at at least 6 degrees at lower speeds. For a boat design to run well at 'hump' speeds you really want the cb to be around 58% lwl.

If this is to be a trailer boat, then most of the perceived maintenance drama's involved with sterndrives can be avoided. I agree that at 3500kg it's getting too big for outboards, but I'm yet to see a waterjet propelled boat that performs well at low planing speeds. Not that they won't do it, just that they use an extraordinary amount of fuel.

The soft chines will make it easier to build strip plank, but will have a negative effect on performance. You have to weigh up which is going to be more important. I like the lack of a hard chine fwd - primarily because it eliminates chine slap whilst at anchor.

I attach an image of a 40 footer I designed with similar lines to yours. I'll also post a few of version 20 of my trailerable version soon too. It suffers from a few of the problems I've mentioned to you, but more of that later....