Hi Alessandro,

Here is a definition of a hard chine hull taken from an online description:-

“A hard chine is an angle with little rounding, where a soft chine would be more rounded, but still involve the meeting of distinct planes. Chine log construction is a method of building hard-chine boat hulls. Hard chines are common in plywood hulls, while soft chines are often found on fiberglass hulls”.


My own thoughts are that when a plywood skin is placed over a frame formed by a series of fairly regularly spaced flat or slightly curved surfaced bulkheads (also called “formers” by some) of a typically standard plywood model boat construction, the sides and under-surfaces are usually more flat that curved……and where the bottom and side skins join a hard edge is adorned, and is how I define a “hard chine” model boat.construction.

Bob.

This how I

Hi Alessandro,

I have been building and driving Classic Model Power Boats since the early 1960s and have clocked up hundreds of enjoyable hours running them (and at one time racing them off-shore too).

Most of these have been hard chine models - but not all.

Almost every boat I have owned and driven has been built or restored by myself, but I have also been fortunate enough to be invited to drive other peoples power boats for lots of different reasons over the years as well, and this has greatly added to the number and variety of different types I have been able to enjoy and experience.

I have also scratch built a TID tug and restored other larger tugs and trawlers so I have also been able to work with them to know how the different types of hull forms react.

If any of this 60+ years of running (mainly) r/c power boats can be used to assist you in any way, I would be only too pleased to help in any way I can.

Bob.

@zooma

Hi Bob, your opinion, like Chris's, is very important to me, and I will take your advice into full consideration.

You wrote:
"Your hull drawings can all be classified and built as simple “hard chine” hulls that should work very well indeed."
I'm very pleased to hear this, but I ask you: what does “hard chine” hull mean?
Could you explain it to me better, perhaps with a drawing?

You wrote:
"A deep vee-shaped hull (as used on several LesRo and Aerokits powerboat designs) gives good stability and generally gives a dry hull and is less likely to roll."
Very good. As I told Chris, this is pleasing and saves me from having to completely revise the stern.

You wrote: "As a matter of interest, I have never found the angle of the prop shaft to be that critical in real-life performance. I did steepen the angle on my Rapier when I restored it in an attempt to drive the bows a little lower when under power, but I'm not convinced that it made any difference."
This observation is also very important and useful to me, because perhaps I place too much emphasis on the inclination of the propeller shaft.

You've been very helpful, and I hope to be able to bother you again with other questions.

Logically, messages aren't compartmentalized, and each person can comment on the messages addressed to the other.

@ChrisF

Hi Chris.

Please note that your opinion on this matter carries a lot of weight with me.

You wrote: "Your vee at the stern is fine; some of my Faireys have a greater vee—they are better in rougher seas than a flat stern. So both will work; what won't work, of course, is a rounded hull, as used in displacement boats. So a vee is a good start—the overall look of your hull is reminiscent of my Fairey Huntsman 28. Some of my Faireys have parallel hull sides, but some also have narrower sides toward the stern, which I think looks better and gives that classic rounded look on plan."

This is very helpful to me, as well as pleasing. I was thinking of creating other variations with flatter hulls (or less pronounced Vs), but after what you wrote, I can leave the V hull as is.
You've cleared up a big doubt for me, and I thank you.

You wrote: "For my water-jet project, I started with the hull from the Lesro Rapier, which has parallel sides, and modified it so that it was narrower at the stern. I also have designs where the stern is even narrower and the sides are more rounded."
In the Lesro Rapier, the bow line is clearly convex.
I initially designed a convex bow (from the side, I mean), but then I opted for a hull with a straight bow (although very steep), as you can see in the attached drawings.
Which do you prefer aesthetically? Are there any differences in dynamic behavior between these two types of bows, or is it the same?

You wrote: "You don't really have to worry much about weight in a planing hull unless you're thinking about using a lead-acid battery in a small boat! In my experience, they are very buoyant and forgiving."
On the drawing, I set an overall length of about 70 cm.
With this length, the immersed volume is only about 1.8 kg, so the entire speedboat should weigh only 1.8 kg.
I could increase the immersed volume by increasing the scale to a maximum of 90 cm (I can't go any higher).
Lead-acid batteries are certainly out of the question in this type of boat, but the limited weight worries me a bit.

You wrote: "There's a question mark regarding my Huntsman 31 as they tend to sit nose down (but plane easily) due to the lack of support from the flared bow, so I might have to use some ballast in the stern - a bath test soon to check that out."
Is this Huntsman 31 also jet-powered or propeller-driven? If it is propeller-driven, I could suggest the cause.
Besides ballast, there could be another system. This system is used for real speedboats, but I don't know how effective and feasible it is for scale models. It consists of fins at the stern that adjust the trim.

You wrote: "It's not the submerged volume you have to think about, but the opposite!"
I don't want to cause any misunderstanding. The submerged volume calculated on the drawing only helps me determine the displacement and therefore the maximum weight of the components (electric motors, batteries, etc.) that I can fit in the hold.

You wrote: "As for flaws, I can't see much wrong with it, though the bow on some drawings looks a bit odd."
This interests me a lot, but it worries me a little.
Since in this initial phase I want to establish a correct, harmonious shape (not only aesthetically pleasing but also effective in terms of handling in the water), I'd like to understand what you mean.

You wrote: "Don't go too long and narrow."
This is also a key point for me. What do you mean?
Is it too long compared to the width, or the opposite?
Would it be better to widen it a bit, or, on the contrary, to taper it?

The model's length ranges from a minimum of 70 cm to a maximum of 90 cm.

You wrote: "but aim for an angle of 12 degrees maximum, though the odd degree more won't hurt."
Okay, that's very specific information I'll take into account.
Let me clarify something. In my opinion, it's best to minimize this angle. The ideal, in my opinion, would be zero. This way, there's no thrust wasted upward (a thrust component that, at least initially, will push the stern up and the bow down).
Unfortunately, in this type of boat, it's impossible to reduce this angle too much due to the engine and propeller layout.
Isn't that the case? Would you prefer to increase this angle?

Sorry for the many questions, but please answer slowly and only when you have time.

Hello everyone.

I was honestly hoping for a response from Chris and Bob.

From your posts so far, I've seen that you have a wealth of knowledge and experience with planing hulls.
So I'm very happy you've commented, and I intend to respond to your messages individually.

Of course, advice and opinions from other modelers are also welcome.

Hi Allesandro,

Apart from Classic Model Power Boats, I also have an interest in Coastal Command power boats and have built/owned and driven quite a few over the years.

All of them have had a hard chine construction and all of them were planing hulls that drive very nicely and can cope well with comparatively rough water.

Your hull drawings can all be classified and built as simple “hard chine” hulls that should work very well indeed.

A deep vee shaped hull (as used on several LesRo and Aerokits power boat designs) gives good stability and generally gives a dry hull and is less likely to roll.

The Vosper and other hull types of Motor Torpedo Boat and Motor Gun Boat that I have driven are all hard chine construction and all of them drive very well and generate the “white water” of the originals when they are under way, and they have all handled very well indeed too.

As a matter of interest I have never found the angle of the prop shaft to be that critical in real life performance. I did steepen the angle on my Rapier when I restored it in an attempt to drive the bows a little lower when under power, but I am not convinced that it made any difference.

I hope this observation is of some value to your design thoughts.

Bob.

I know the prop shaft is a consideration for later but aim for an angle of 12 degrees maximum though the odd degree more won't hurt.

The Huntsman 28 was my first build and I didn't want to cut the prop shaft and tube any shorter so the motor is further forward than it needs to be to achieve the required angle. The angle/attitude of the model on the water is fine though as the weight of the motor is easily offset by the other components.

But hull design first!

Hi Alessandro

I don't do naval craft, so does this exclude me from the discussion! Only joking!

But I do do planing hulls! As you say easier to design than displacement or semi-displacement. As long as the hull has a flat or vee at the stern and sufficient power it will plane!

Your vee at the stern is fine, some of my Faireys have a greater vee - they are better in rougher seas than a flat stern. So both will work, what won't work of course is a rounded hull as used in displacement boats. So a vee is a good start - the overall look of your hull is reminiscent of my Fairey Huntsman 28. Some of my Faireys have parallel hull sides but some also narrow towards the stern which I think looks better and gives that classic rounded look on plan. For my water-jet project I started off with the hull from the Lesro Rapier which has parallel sides and modified it so that it was narrower at the stern - I also have designs where the stern is even narrower and the sides more rounded. Depends on what your overall design for the model is really as to what will work the best.

You don't really have to worry much about weight in a planing hull unless you are thinking about using a lead acid battery in a small boat! In my experience they are very buoyant and forgiving. I was a little worried about the stern drive on my little 23" Huntress but it sat bang on the waterline and performs really well. I don't build light and so far all my models have sat on the waterline. There's a question mark as regards my Huntsman 31 as they tend to sit nose down (but plane easily) due to the lack of support from the flared bow and so I might have to use some ballast in the stern - a bath test soon to check that out. It's not the submerged volume you have to think about but the opposite! As I said, very bouyant!

Motor will go towards the front but then if you distribute the battery in the middle and the rudder and prop shaft obviously towards the rear you can't really go too far wrong.

As for flaws, can't see much wrong with it though the bow on some drawings looks a bit odd. Don't go too long and narrow - I've noticed with a number of MTBs/crash tenders/fireboats models that they can be prone to dropping too quickly into turns and are a bit unstable, partly in some cases due to the operator not being smooth on the rudder and not helped by the relative lightness of the model.

What length are you looking at?

Chris