Playing around with a 10 m trailer cruiser

[rwatson]

If you bought Mac with no mast you would have something you can pull with your car, and have all the comfort and speed you expect from your own design.

As far as how much cruising you can do - join the other 8000 people with the same idea. Only you won't have a sail to waft along on gentle breezes as an option to the gas guzzling, carbon emitting, dollar gobbling monster sitting on your ***.

I am not trying to push Macs, by the way, just using it as a benchmark I am familiar with.

I am planning a re-designed Mac myself -

http://www.boatdesign.net/forums/showthread.php?p=227973

with an inside Nav Station. Whats the point of sitting in the rain as you motor along ? (as well you already know)

All banter aside, are you putting in water ballast as an option?

In really rough weather it made the ride so much more comfortable, and with the help of collapsible freshwater tanks, the space doubles as a huge freshwater supply.

 

[Willallison]

Almost regardless of the method used, the figures I ran through for my own boat suggested I would need about 150 shp in order to attain a top speed of 25 knots, with a displacement of 2800kg. On that basis, I was originally going to opt for Yanmar's new 4BY, which comes in 150 and 180 hp variants. In the end I chose to opt for the 6 cylinder engine (6BY) to take advantage of the massive inc, so your figures seem consistent with thatease in torque available. This engine is available in 220 and 260 hp. I'm telling you stuff you know, I realise, but it can sometimes be a mistake to opt for smaller engine. A bigger, lightly stressed one will often return better fuel consumption, last longer and offer more consistent performance - particularly if you have a varying payload as cruising boats often do.

As far as the 8m / 2 tonne cruising boat goes, why not? My family cruised for years on boats of that sort of size - and there were 5 of us. The weight issue will really be determined by the level of amenities you (read "the wife") require. In our case, we spend quite a bit of time in the water, with two small kids and a couple of dalmations, so a shower was deemed a necessity. That pretty much determined the pattern for the rest of the boat.
If the 4 people on board can maintain the less-is-more cruising lifestyle, then there won't be a drama. Sway just a little bit though, and you'll be glas of a bit of extra grunt!

 

[marshmat]

If I could afford that Yanmar 4BY, Will, that would be my engine of choice for sure on the 8 m boat. I actually used the 6BY in some of the initial estimates on the 10 m. However.... I haven't been able to get a straight answer on the price of the thing, other than it's probably well north of ten grand- and our dollar's high right now. Hence the desire to move down to the 3.0 I4 or 4.3 V6, either of which can be found with a four-figure price tag in this area.

The engine debate, of course, won't be settled until construction begins- at least a couple of years away. Maybe the 4.3 V6 (~190 hp) will win out.... but fancy diesels are just out of the price range for this particular project.

I hadn't thought about water ballast, but rwatson has a point- and the model tests done so far do indicate, qualitatively, a much smoother motion in waves at higher displacements. (We can load the model down to about five tonnes scale displacement, although it's a bit reluctant to plane like that.) This will get some further thought.

I would love to have a sailboat. Indeed, I wouldn't be surprised if a small one in need of a lot of TLC mysteriously shows up in the driveway in a few years. But anything with a mast is an absolute no-go for this particular project; there are just too many low bridges or timed-opening bridges. Stepping and unstepping the mast three times a day, even with a fast-folding rig, would get tiring in short order. And it is looking increasingly likely that this boat would be drafted into handling some serious cargo and/or towing duties on occasion.

I think that this time around, amenities can be kept to a minimum. This summer's Rideau cruising consisted of pitching a tent on shore alongside Sunset Chaser, and using the lockstations' facilities. That was a lot of fun, even though SC is open and it rained. The ability to sleep on board, plus a basic head and small galley, ought to suffice for the new boat. At least, until it comes time to build another, larger, boat.... but that is another story.

 

[FAST FRED]

"but it can sometimes be a mistake to opt for smaller engine. A bigger, lightly stressed one will often return better fuel consumption, last longer and offer more consistent performance - particularly if you have a varying payload as cruising boats often do."

This is perfectly true for GASOLINE engines , and the many auto transplants/marinization of diesels.

However it is Not True for a genuine rated diesel, where the higher load equals higher (sometimes 300%) efficiency.

Take a look at the Cummins B4 , there old cheap, reliable workhorses that can be purchased rebuilt at a fraction of the cost of a BMW , Yanmar marinization.

FF

 

[Willallison]

Quite right FF - though there's generally a comensurate weight penalty to be paid if you want a commercially rated engine. In effect you are suggesting much the same thing as I was - a bigger lightly stressed engine often represents better value than it would at first appear.

 

[marshmat]

In the search for distractions from this week's crystallography and condensed matter physics work, I'm trying to spend as much time as possible messing around with boats instead....

Spray rails and gunwale rub rails have been changed to aluminum. I am getting sick of touching up the paint on Sunset Chaser's wooden rails every time I have to dock at a less-than-perfect pier. (And, sadly, I had to lock her up for the winter yet again last weekend.... seven seasons and 175 trips later, she's still going strong.)

When I get a chance to sit down for a few hours and run some more numbers on this thing, one possibility I am considering is to increase the beam of the 8 m boat somewhat- keeping the same overall shape, but changing the proportionality of the L/B/T scaling factors to give a somewhat wider hull in the little one (approx. 2.3 m max beam, up 20 cm from what is currently specified). Dry weight would be kept the same and the boat would ride slightly higher when empty. The roll period would be a little bit snappy, but I don't think that's a huge issue in the kind of weather and sea states being considered here.

Tom Lathrop managed to get his Bluejacket light enough, and with a low enough bottom loading, to climb through the planing 'hump' with virtually no fuss. I'm expecting this boat, properly balanced, to behave in a similar fashion- climb onto plane early, with relatively low weight for the available planing surface, and not exhibit significant bow rise / stern squat / canoe-tipping-wake-production at the intermediate speeds. My 8 m boat would be about 10% longer than Tom's and 70% heavier, as well as being soft chine, so I'm not expecting planing efficiencies as good as the Bluejacket. But my calculations so far indicate that I'm on a good track for the intended purposes.....

 

[marshmat]

The lake is frozen
And my test model is tied to the ceiling of the garage.
Oh, and I had to chisel the car out from an ice sheet last week, after shovelling probably 1200 pounds of snow out of the way to get to said car.
Stupid Canadian winter.

Oh right, boat.....

So, the race is now on to see what gets built next. I'll be done with the university in a few months and, hopefully within a year afterwards, will have some space to start building again.

The 8 and 8.5 metre versions of the hull discussed here are leading contenders. I'm hoping to do some more tests with the model when things thaw out a bit here. Performance is looking promising, and I'll be comparing ways to build such a hull lightweight and cheap. Power on either the 8 or the 8.5 would be a 4.3 V6 or 3.0 I4, probably with an Alpha or Volvo SX (it doesn't look like the HJ jet pump will stay within budget).

The challenger is a trailerable powercat of similar size, that would probably be fitted with twin 20-25 hp outboards. (These can be found,used, for $1500 a pop right now... makes it very, very hard to justify anything more elaborate, even though I would love to do an electric boat.) For a marked improvement in fuel economy, this option would sacrifice living space and a bit of cargo carrying ability. And build time would likely be longer than for the planing mono.

In either case, I'm going for maximum simplicity. Minimal systems, no superfluous electronics, and skip the ultrasuede settee with birds-eye jarrah trim. I want to keep build time to a minimum.... partly to get out on the water for some longer cruises, and partly to free up space for future projects....

The other challenger.... well, I want something with sails around, simply because sailing is fun. And there are a lot of (bad) concepts scattered all over my drafting board right now, if you dig through the stacks of quantum calculations that have been breeding around here lately. But what's wrong with having both a powerboat and a sailboat, right?

 

[tom28571]

Matt,

I don't know how I managed to miss this thread for so long. Getting too lazy to check out the forum often enough to see it, I guess. I really like the look of the early hull forms as well as the early superstructure designs. The hard chine satisfies my needs and desires better than the round hull though. It's a compromise, of course but so far, I see no reason to change for a planing hull. I think that quietness at anchor is the major advantage of the round bilge. No boat is really comfortable at speed in rough water whether it's a deep v, round hull or catamaran. Only moderate sized cruising boats with good accommodations considered in that last remark.

I have not had time to look at all 10 pages of the thread and am certain to have missed some important iterations of the hull. Some later drawings as well as the model seem to have parallel aft chine lines rather than the reduced transom width of the earlier lines. Combined with a fairly fine entry, I would think this is a mistake and may give problems downwind in waves.

Towing video: Is the bow line loose enough to allow freedom of motion in yaw and vertical bow surge? The running video shows the trim angle to be much greater than I would want. I think it was Will who said the water is hitting the hull bottom too far aft where it will cause slamming. He is correct. The lines show a boat able to run at multiples of hull speed but the running video shows that it will probably have a fairly large planing hump. I'd guess that speeds from about 7 to 15 or 16 knots to be a no go range. The reason is probably the weight and/or longitudinal CG location too far aft. You can move the CG forward, reduce the weight or add some stern lifting hull feature. I'd want all of these but that is for a boat that will run at several times hull speed at low power.

Right this moment, I can't remember just what your intended speed is.

I know this is a rambling note but hope all will excuse me and just mark me as being a late entry here.

 

[tom28571]

But what's wrong with having both a powerboat and a sailboat, right?

Wonderful, as long as they are not the same boat

 

[marshmat]

Good to hear from you again Tom

In the model runs I did in the summer, we left the bow line essentially slack; it's only there to stop it from spinning around while docking the big boat. The model was towed and steered by the side bridle lines, which led to the same point on the towing arm (about a metre ahead of the model). This seemed to work very well.

I think the video I posted was from early in the test, before the camera ran out of space. I agree with you and Will that we had the CG too far aft that time, hence the bow-up tendency and the slamming. Savitsky calcs suggest that for the 8.5 m boat- the one all the design work is being focused on right now- the CG can't go any farther aft than 36% of LOA forward of the transom, and ideal seems to be a hair under 40%. CG was at about 37% of LOA in that video, IIRC, so the model would appear to confirm this. But all that was done at a fairly heavy displacement- 2370 kg, and 50 ihp @ 20 kt / 70 ihp @ 25 kt (about a 130 bhp engine).

Trimming the displacement to 1800 kg, in the Savitsky model, brings us to plane at 12-13 knots with not much of a hump. Dropping to 1250 kg pretty much eliminates the hump altogether in that simulation and has us hitting 20 knots on 34 ihp (about a 50-60 bhp engine)- I'm aiming for 20 knot cruise, no-wake at 5-6 knots, and low to mid 20s at the top end. I can't drop the model's scale displacement this low right now, because it's too heavy- next season I'll probably try to cut a bunch of material out of it to lighten the thing.

I've posted a few screenshots of the lines of the 8.5 m version as it stands now, waterlines drawn at 2800 kg which is about as heavy as it would ever be run. They're not in the proper linesplan format, but this late at night I don't think it matters.... you guys will understand it just fine.

 

[tom28571]

Good to hear from you again Tom

In the model runs I did in the summer, we left the bow line essentially slack; it's only there to stop it from spinning around while docking the big boat. The model was towed and steered by the side bridle lines, which led to the same point on the towing arm (about a metre ahead of the model). This seemed to work very well.

I think the video I posted was from early in the test, before the camera ran out of space. I agree with you and Will that we had the CG too far aft that time, hence the bow-up tendency and the slamming. Savitsky calcs suggest that for the 8.5 m boat- the one all the design work is being focused on right now- the CG can't go any farther aft than 36% of LOA forward of the transom, and ideal seems to be a hair under 40%. CG was at about 37% of LOA in that video, IIRC, so the model would appear to confirm this. But all that was done at a fairly heavy displacement- 2370 kg, and 50 ihp @ 20 kt / 70 ihp @ 25 kt (about a 130 bhp engine).

Trimming the displacement to 1800 kg, in the Savitsky model, brings us to plane at 12-13 knots with not much of a hump. Dropping to 1250 kg pretty much eliminates the hump altogether in that simulation and has us hitting 20 knots on 34 ihp (about a 50-60 bhp engine)- I'm aiming for 20 knot cruise, no-wake at 5-6 knots, and low to mid 20s at the top end. I can't drop the model's scale displacement this low right now, because it's too heavy- next season I'll probably try to cut a bunch of material out of it to lighten the thing.

I've posted a few screenshots of the lines of the 8.5 m version as it stands now, waterlines drawn at 2800 kg which is about as heavy as it would ever be run. They're not in the proper linesplan format, but this late at night I don't think it matters.... you guys will understand it just fine.

Yes Matt, That is one reason I put the slack line to the bow as an aid in getting the model started without running over the tow line. The main reason is that at high speed in warped bottom configuration, the model yawed so violently that it swapped ends and tore up my towing rig. I intend to investigate this further.

I don't know what the driving variables of the Savitski model are but I found that highest top end and best hump performance are not compatible. That is, the trim angle to give best speed is too high for early planing and minimum hump. I opted for minimum hump and level running which also gives best comfort in chop with the bow down. That is why my CG is further forward (more than 42%) and why there are very high lift chines aft. This has worked well from 24' to 27' LOA and waiting on results from 28'.

I think test models should be built lighter than scale design values to allow variation both higher and lower than nominal for testing the hull on a broader range of weights. I went from -20% to +80% of nominal. Models don't need to be built so strong. 3mm ply with hardly any interior structure should be plenty tough for testing. 12lbs should not be hard to attain in a 5' to 6' model.

Guess I don't understand why your design displacement is so high for a 7M LOA boat. It will be harder to get good low speed planing performance with bottom loading that high. I'm guessing that the scale bottom loading in the video is over 100lb/ft sq. That is not so high for many existing boats but far too much for eliminating the hump and efficient low speed planing. Less than half that should be possible for your boat and still have a robust and well equipped boat.

These are not criticisms, just thoughts that have worked out for me.

 

[marshmat]

Hi Tom,

Thanks for the comments. You've been through this process, evidently with considerable success, and I appreciate your input.

I found that highest top end and best hump performance are not compatible. That is, the trim angle to give best speed is too high for early planing and minimum hump.

I find this too. Speed potential requires the CG to be brought aft, thus causing squatting in the "hump" range. A hair under 40% of LOA seems to be the ideal spot for this hull. From the drawings I've seen of your Bluejacket, it's a little fuller in the bow sections and so I'm not surprised to hear that you set your CG a bit farther forward.

I think test models should be built lighter than scale design values to allow variation both higher and lower than nominal for testing the hull on a broader range of weights. I went from -20% to +80% of nominal. Models don't need to be built so strong. 3mm ply with hardly any interior structure should be plenty tough for testing. 12lbs should not be hard to attain in a 5' to 6' model.

Had I known this before starting the thing..... lol. The model I built for this hull is two layers of 10 oz glass over 2mm birch. The weight (12.9 kg for a 1.67 m model) comes from the heavy Masonite frames and deck I used, which I will probably cut out next season so we can test lighter displacements.
If doing this again, I would probably build a plug of XPS foam and glass over that, then remove the foam. Quicker and lighter than how I built this one.

Guess I don't understand why your design displacement is so high for a 7M LOA boat. It will be harder to get good low speed planing performance with bottom loading that high. I'm guessing that the scale bottom loading in the video is over 100lb/ft sq. That is not so high for many existing boats but far too much for eliminating the hump and efficient low speed planing. Less than half that should be possible for your boat and still have a robust and well equipped boat.

The 8.5 m boat has a wetted surface of 14.7 m^2 at rest, somewhat less (50 to 70%, perhaps) when on plane. At fully loaded displacement, 2370 kg, that's somewhere in the 200-300 kg/m^2 (40-60 psf) range for bottom loading when on plane. That's fully loaded- six crew, all their gear, full tanks, stocked cooler, etc. or perhaps a full-size pickup truck load of building supplies, ie. a worst-case scenario for attempting to plane. Bottom loading on the new boat in light operating condition (115 kg/m^2, 24 psf by at-rest wetted area when displacing ~1600 kg) is actually about the same as for my Bolger Diablo with one person aboard- the Diablo, properly balanced, is fully on plane at about 10 knots.

So yes, she's a fair lot heavier than the Bluejacket... but also is less than half the weight of a lobsterboat form in this length; I looked at those before starting the project and most of this length were in the 4-5 tonne range loaded.

 

[tom28571]

Ah so Matt, those numbers give loading factors of about the same as mine. Once I thought of going lighter for academic interest but considered it not too practical. Big and lightweight can offer some problems docking in strong crosswinds. Other than that, I have not found any disadvantage to the lightweight boat.

I use a static/planing factor of .67 which is probably a bit lower than reality to be conservative. Since the buttock lines on both boats are level to the static waterline I would shoot for a trim angle of about 2 degrees max. This will have the stem clear of the water but not by much. That will make for easier wave entry. Bluejacket's bottom forward is a bit fuller than yours but it is a compromise between having a fine entry and getting adequate room forward for berthing comfort. I am a bit suspicious of the size of the forward berth in one of your drawings. I suspect that berth will have to be pretty high off the bottom or further aft to get the foot room shown in the drawing.

I am not sure that a 40% location of CG will get the bow down as much as you may wish. The soft chines aft will have less lift than hard chines and there are no wide aft chine flats either. I've known some similar hulls to have chine flats added to get more aft lift. The round chine hull has been proven more seaworthy at higher speed than hard chines, as witness the German E boats versus British and American patrol boats of WW2. For small and slower inshore boats, I don't think the round hull pays its way well enough. It's wetter and harder to build also.

 

[marshmat]

The forward berth in question was sketched very quickly in the back of a van somewhere on a bad stretch of Hwy. 11. I will, eventually, get around to drawing an interior that fits in the boat

I tend to agree with you, Tom, that chine flats would improve the planing efficiency somewhat. I'm reluctant to use them here, because this boat- although primarily an inshore cruiser- has to be capable of handling some rough stuff on Lake Ontario, Georgian Bay, Lake Simcoe, etc. at off-plane speeds, and I'm afraid that chine flats big enough to make a difference would lead to a snappy roll and a bit too much pounding on this light, shallow-V hull. I'd like to use the underside of the spray rail to get a little bit of the chine flat effect without the snappy roll problem, although this would certainly not be as efficient as true chine flats.

My current Bolger "Diablo" is about as light and high-sided as 5-metre runabouts come, and she can be quite a pain to dock when unloaded. Bringing her alongside in a breeze is a lot like trying to run a slalom course in a Sunfish with the board up. I'm not terribly concerned about the new boat being harder to dock than this!

The temptation as far as CG is concerned is, if this boat gets selected for construction, to do a few sea trials with the hull, engine, portable gas tank, and sandbags, before finalizing the tankage, interior, etc.

 

[tom28571]

I will be anxious to see your boat finished and on the water. You have to quit designing sometime

------------------------

In my case, I drove as many boats as I could, carefully watched the many boats running around this coast, talked to all who would take the time, studied and calculated and drew lines and studied and calculated and drew more lines for several years. Finally, in desperation, my wife said "either build that boat or throw all that stuff out". Exactly the advice I needed. When a truck showed up with a stack of plywood, the commitment was final.

You can see the opposite of this approach at: http://www.woodenboat.com/forum/showthread.php?t=90321

This is a boat being built by a friend nearby. Very little in the way of paperwork or calculation but backed by building and using many boats.