CONTINUOUSWAVE

PURPOSE OF THIS THREAD

The 13 footer was developed in 1957 and went into production in 1958, and planing-hull technology and engine technology as changed since then. s. When the original 13-foot hull was being developed the highest power outboard engine in production was the Mercury Mark 55, which was rated at 40-HP. Mercury later introduced the Mark 75 (60-HP) and OMC came out with their V4 50-HP. Both engines were too heavy for a Boston Whaler. Later, engines of 50-HP (like the Mercury 500) were being used on Boston Whaler boats for the last 58-years.

A older post describes the experience of running a Mercury 500 on Boston Whaler 13-foot hulls. While they say boat isn’t stable [at the speeds produced by a Mercury 500], it’s being done all the time.

I’ve found all the posts on the subject of the elimination of a hook in the hull, but there is precious little actual information on what was actually done and what the results were for each modification. I’ll try to summarize:.

I know that the boat as originally designed and molded becomes unstable at high speed.

I want to define set of modifications that work and improve the boat. I hope for replies that have positive based on experience or technical approach, or have any positive ideas. The idea of trying to go faster than 50-MPH in a classic 13-footer is not the object of this thread.

I fulfill my ned for speed with an APBA “D” class runabout that will do close to 80-MPH. I am not trying to make a 13-footer into a race boat. I hope to collect information about modifications make sense in light of modern technology.

With the the 13 foot hull there should be improvements in speed, stability, ride comfort and efficiency available through hull modifications in combination with anti-ventilation plate foils, power trim, and trim tabs.

THis thread hopes to define what modifications are worthwhile and what changes are a dead end.

The original 13-foot hulls runs very flat because of the hook, and once you get moving fairly quickly, even with 30-HPp, the bottom is all wet and the boat rides much rougher and has higher drag than [a boat that might be designed today for] day’s technology.

TECH TALK and SUMMARY of WHAT’S ON THE WEB

The 13-foot hull can be set up to run in about 45-MPH with a 50-HP engine--if increase trim-up to overcome the hook that was built into the boat to help get it on a plane with a small motor. Trimming out the engine really just pushes the transom down and with the hook in the bottom you end up running nose high on a very short bit of bottom, with the hull trapping air and very little of the boat in the water. This is basically an unstable operating condition, and if you try to turn the boat, since the strakes and almost all of the keel iare out of the water, the boat yaws and doesn’t turn until the hull slows down a bit or some of the power that is holding the bow up is pulled back. Then he keel drops down and the strakes get into the water and bite, and the boat violently hooks in whatever direction it is pointed. The net result can be throwing the occupants out or rolling the boat over. The nose high condition also traps a lot of air under the boat and you have the potential to blow the boat over. That’s not as common as rolling it, but it remains a possibility. So running speeds in the 40-MPH range with an unmodified 13-foot hull isn’t a good idea--we can all agree on that.

The hook in the bottom starts about four of five inches forward of the transom, and is about a half inch in height, on both sides of the keel. Some folks have filled in the hook, but unless you fair it out over a long distance you actually change the hull line. Even if you get a flat hull bottom, you now have a bottom that isn’t running at the same angle of the keel. So, when the boat is running high on the aft part of the new surface, the keel forward isn’t as high as it would be if the hook was removed by cutting it off. Filling may be better than not filling, but it isn’t optimal since the nose isn’t as high off the water as it should be.

User mgajim reported filling the hook and also trimmed off the two fins on the bottom. His experience was that the boat didn’t turn well, that is, it skidded more with the fins cut off at speed, but that’s not unexpected given that there isn’t much else to grip the water and you need something to help you create lateral forces. He didn’t experience any tendency to porpoise that is also interesting, and he reported that the boat didn’t pound as much with the hook removed. The reason for the reduced pounding is that with the hook gone the boat is running at a higher trim angle and this allows the transom to go a bit lower in the water and the boat to have a higher bottom loading, all good things for reducing pounding. That improves the ride significantly, so there’s another example of an improvement that isn’t necessarily related to speed or efficiency, but was an improvement that any user could appreciate. There’s a list of the other posts on the topic in that thread and I’ll skip relisting it here.

Other than the above, not much else is out there relative to what has been tried and what works so we can talk about that in future responses, but first I’d like to try to capture as much information as that is out there and then go from that to a rational approach and then go out and test some stuff. The boat I have is old and isn't really in nice enough shape to be a candidate for restoration although I do have all new wood in it. I expect to do some calculations on some theories I have on what might work, but need to refine that first.

Your investigation into modifications to the running surface of the classic Boston Whaler 13-foot hull is bound to be interesting. I look forward to hear more from you about your experiments and from others who can contribute to this discussion.

jimh wrote:Your investigation into modifications to the running surface of the classic Boston Whaler 13-foot hull is bound to be interesting. I look forward to hear more from you about your experiments and from others who can contribute to this discussion.

Thanks, I have a Savitsky analysis spreadsheet that can determine things like running depth, trim angle (angle of the hull with respect to the surface), power and porpoise stability, so I can look at things like what a pad would do, but the analysis is somewhat limited in that it can't combine the effects of a pad with the hull if the pad is fully immersed and the hull is running part on the pad and part on the hull. This will tell us how fast the boat could go, but it doesn't have any info relative to lateral stability.

I also have the ability to model mods in CAD, but I'll have to do some work to get the Classic 13 bottom into a model that is accurate enough to work with. A CAD model would help visualize any mods, and that helps make sure the analysis inputs are reasonable.

The biggest concern is actually the stability that goes away when you start going over 40 mph with the motor jacked way out. My thought is that if you could get the hull up a bit on a pad, get rid of the last bit of hook and modify the fins that you might be able to avoid the problems that others have noted with the boat grabbing and even rolling if you tried to turn at speed. By adding the pad with vertical surfaces that are about 2 inches or so on the pad you'd still have about the same surfaces for turning, but the boat would likely respond more like a conventional V or padded V hull and not exhibit the turning problems the current hull has at speed now.

What is interesting about the analysis is that if you look at what a modified bottom could do at speed, the efficiency of the hull in the 30 to 40 mph range can be significantly improved. For example, if you ran a padded hull with a pad width of about 11 inches you'd only need about 25 hp to run 40 mph... That's assuming a 70% prop efficiency, but that's probably a realistic number..

What this says is that you don't have to go 60 mph to see some advantages from a properly designed mod.

As I work on defining what I think will work I'll keep you posted.

Any updates on this? I just picked up a 1968 SPORT 13 that had no real hull information—hull number was painted over. So I’m going the resto-mod route and looking for ideas.

I Was planning on removing the hook, but I was thinking about what to do with the fins. The idea of a pad bottom is intriguing, but I still want the boat to be relatively stable.

Looking forward to updates.

I've been running my 13-footer with [a late 1970's Mercury 50-HP two-stroke-power-cycle four-cylinder outboard engine]. This engine isn't all that light, but it's a lot lighter than most modern motors. I don't have power tilt or trim, so that takes a bit out of the equation. I've run a couple of different propellerss and can easily push the boat into a range where it plows and becomes unstable. If I had power tilt and was able to have the engine jack way out then you might be able to get it to run more comfortably, but I'm not sure. At that point you're riding on just the [hull] hook and that doesn't sound very stable to me.

I did some measuring and some calculations and a 12-inch pad that was level with the bottom of the keel would support the boat at speeds over 45-MPH. The area outside of the pad would just barely be touching the water, so [the boat] shouldn't do a lot of chine walking, but would be pretty efficient. This would take the hook out of much of the middle of the hull so then you'd only have to remove the hook from the areas between the fins and the pad.

I'm in the midwest and it's a bit too hot to work out in the garage, but the plan is to mod the bottom of my boat in the fall and over the winter and then test in the spring.

As to the fins, you most certainly don't need that much fin. However, one person in another thread took the hook out and cut the fins off and wasn't happy with how the boat handled, it skidded too much. The bottom line is that you need something to keep it from skidding. If you had a pad you'd have a good size step that would act like a fin, preventing skidding. Padded hulls don't have fins and turn just fine, so if you go with some kind of padded design you can cut off the fins. If you just take the hook out, then you're going to need some kind of fin to prevent skidding. Part of that equation is how you get rid of the hook.

If you try to fill the hook area you need to carry that flat surface a long way forward. If you don't you'll end up with a nose down trim angle and at high speed you'll be back with the bow plowing and instability. Much better would be to use a pad and then cut the hook out on each side of the bottom near the transom and then replace the surface you cut out with a flat surface. That's pretty much what I was planning. I'll do some measuring and make up a CAD model of the bottom before I start hacking at the bottom so I can see what it's going to look like before and after.

Please share your process. I just started getting into the transom repairs yesterday and that is going to more involved than originally anticipated so it will be awhile before I flip her over and start working the bottom.

Also any type of modeling, drawing, or data that is avaliable on the original hull designs would interest me. I like old drawings on this kind of stuff.

I spent an hour or so on my back under the transom on a creeper doing some preliminary measurements and figuring out what the mod is likely look like. What I will do is run a Savitsky spreadsheet that can predict performance for various planing surface widths and come up with a width that will get up most of the way on the pad, but I'm not trying to make this into race boat, I just want it to run mostly on the pad, but I don't want to have any problems with chine walking or lateral instability. Once the hook is gone that will be 90-percent what I'm looking for, and you could do that without the pad, But the pad will add some efficiency, and if you shape it to the front to V and blend in with the existing hull it won't pound as much.

What I could see under the bottom is that the keel has a bit of rocker in it as it approaches the transom. This is partially obscured by the hook in the bottom. But if the keel line was extended it would be pretty closer to right where you would want it to be relative to the lower unit. What I mean by that is that if you make a pad that is deeper than keel line you're going to need to lower the motor more or get a surfacing prop. So the plan is to design a pad that is about a foot wide and is in line with the keel. Then to the sides of the pad you need to remove the hook, which means cutting the hull with a buzz wheel and removing the hooked bottom, flattening out the foam and re-glassing the hull in the area between the pad and the skegs. I'll carry the pad a good way forward and eventually v it so that it doesn't pound, but the last 4 feet of the pad will be flat and the pad will be square or perhaps have a very slight V (less than 5 deg) of deadrise. A very small amount of deadrise will help the boat roll in turns. Of course there won't be much roll before the chine on the inside of the turn hit the water, but you'd like to get a bit of roll when you're at speed.

There aren't any drawings or models of the bottom of a whaler that I know of. I could model it, but the biggest problem with that is the time required. You have to draw lines across the hull and then measure from a datum at a lot of locations and then create a grid of points and then evolve the surfaces. If you were going to do CFD on the hull that would be fine, but it's not really necessary to create a pad. You just have to go in and make sure it's all straight. It's more of a building job than a modeling job, unless you're going to do a bunch of high power analysis and that's really not necessary for the goals I've set.

[This thread has been closed to further replies after it was dormant for five years.]