Stretching a hull

I've been doing some more math and it seems I've made a mistake with regards to the necessary length of the extension (way to many numbers floating around...).

In total it ads up to about 3100 mm (with the possibility of reducing it a little if I accept less space between the different components). The L/B ratio will increase from 2,93 to 3,96.

The weight of the extension will be approximately 920 kg based on the hull and cabin surface area with a 5 mm hull thickness and 4 mm cabin thickness. 2,7 as the aluminum density and an increase in material (for supports and such) by 50 percent. I know, a rough estimate but in the ballpark.

Based on the estimated volume (assuming a 14 degree deadrise) of the hull this means that the extension will be submerged to the chines in order to carry itself. Adding 1200 kg (the weight of the engine and transmission) will put it 125 mm lower (in reality less as the extra mass will be carried by the entire hull). Given that I'll also remove 800 kg from approximately what is currently midships (and add a heavier prop shaft and prop) it would seem that the boat will be more "stern heavy" which is in line with what I'm looking for. But not by an unreasonable amount.

With a drive shaft length of 2200 mm the transmission will be placed approximately 1000 mm ahead of the current transom. This would be beneficial as it would clear the current fuel thank and thus saving me the trouble of building a new one.

What are your thoughts?

 

Maybe you will drive the boat faster with the bigger engine and the longer but also heavier boat, and as a consequence burn more fuel.

Do the old tanks have enough capacity for the desired range after the modifications . . ?

An extra tank (or maybe tanks) somewhere would also give a weight trim option while burning the fuel, and it would give the option to adapt the weight trim to the ever changing freight loading conditions.

 

I'm not sure how much gain I'll have in top speed due to the added length, from my experience lwl seems to be more of an interest in lower speeds (I'm talking about small planing craft here, i.e. boats).
I seems reasonable that the slender hull will be more efficient in deplacement speeds, compensate for the increased mass at lower planing speeds (i.e. be on par with current net efficiency in the planing sub 20 knot range) and then start to burn more (than the current hull would have) the faster I go.

However, I also think that the new design will be at it's peak net efficiency at a higher speed than today (time is also money).

 

Update.

I've been talking to several boatyards about my project and while some are interested none have been able to fit me into their schedule (I need it to be finished in late August and they want to do it in the winter but that's when I need the boat).

So the decision has been made to replace the current Mermaid (Ford) and PRM601 transmission with a FPT N67-450 and a PRM1000 transmission. The engine will be downrated to 350 hp, which puts it at medium duty, and it should provide a nice boost in performance (+90 hp).

I'm not very happy about the transmission. The PRM1000 is a direct swap for my 601 (they're the same size and the 1000 is an updated 601) so it'll fit without any modifications (very tight space). However, the 350 hp output at 3000 rpm puts me close to the PRM1000's maximum capacity which I do not like. Another issue is that the engine flatlines at 350 hp from about 2100 rpm to 3000 rpm which further increases the stress on the transmission at what will essentially be cruise range rpm (or am I missing something?) as PRM rates their transmissions in hp vs rpm. The clutches are the weak link so I'm hoping good oil, frequent oil changes, proper temperature and sensible driving will keep it happy. I'll also rebuild the current 601 so I'll have a spare.

I will still be doing a hull stretch but only a modest one, about 70-80 cm, but since prop and rudder will remain in the current position it will be a fairly straight forward operation. I'm doing it in part because I need to redesign the rear of the cabin and I need a bit more space. But mostly it's because I need a new transom shape since the sharp corners makes it almost impossible to reverse in ice.

 

About the transom.

The current design is as shown in the picture and the way I see it there are two possible design choices – one with round "corners" and one with 45 /45 degrees (also I'd like to keep the new transom "flat"). And just to be clear the pics are not to scale, only to show what I mean. I'm not sure how much corner cutting I need but approximately a 150 mm radius seems reasonable.

I've seen both design on planing hulls so I guess they both work. But which one is preferable as far as performance goes?

 

Magnus
Since the chine is probably not dry, I would choose version 2. In this way, the water flow will release better at the sharper corner than the rounded.

JS

 

Hi Magnus, if you only mean here you're close to the PRM1000 transmission's max Hp capacity, and not torque and Hp, then the FPT N67-450 engine downrated to 350 Hp might go over the PRM1000's max torque capacity, at the max torque level in the downrated engine's rpm range, best check this I'll think...

Godspeed with the project !

 

That would also be easier to build, I'll think.

 

The idea with most "faceted" chines is to help prevent "tripping, in high speed turns. These can be done a number of ways from fully rounded over with a generous radius to a simple bisected angle across the chine. If the boat can do more than say about 35 MPH, consider the bisected chine, which will cut a cleaner path at speed. If on the other hand your boat isn't a speed demon, consider rounding the chine, which will also soften the ride a touch.

 

My guess is you have a misinterpretation of the post #35 sketch Paul, I'll think it shows a top view of the old and new transom before and after the stretch, and not an aft view or cross section of an upside down hull eg during construction. The in top view 90° corner angles of the current transom are bad for going backwards in a field of ice floes, so that needs to be improved on the new transom of the stretched hull, hence the suggested rounded or 45°/45° bisected transom corners.

the below is my interpretation of the post #35 sketch

 

Hi Jürgen, I've just looked at some of Magnus' pictures on these forums, in rest (and being unloaded ?) the whole chine looks to be just above the water, I'll think...

Pic from post #1 of this thread:


Pic #2 of post #1 on the thread ‘‘Hi all from Sweden’’

 

I'll guess this means that the corners of the V-bottom to the transom underside also need to be rounded or 45°/45° bisected in the new situation, for enhancement of the reverse possibilities in ice floes, since it looks like it's mostly the V-bottom that sticks into the ice floes . . ?

If so, then I'm afraid that at the V-bottom to the transom underside transition you'll sure lose some water flow release efficacy vs the old 90° angled transom corners there in the current situation.

 

The chines are dry at speed (I think). While I don't have a photo of it I would very much suspect so since, as Angélique pointed out, the boat "sits quite high in the water" (for lack of the correct term).

Does the term chine also apply to the transition between bottom and transom?

My sketch does refer to the transition from topside to transom. Is there a term for this part too?

If I decide to go for the bisected design – while not as good looking it would surely be easier and on par with the rest of the design (or lack thereof) – I have a few questions. I know I can only get a vague and general reply but it's something to begin with.

1. Is 45/45 degrees reasonable? As far as ice goes it would be best.

2. How big can this section be? From an ice point of view "the bigger the better" but ice is only a fraction of what needs to be taken into account. As it is today the boat needs constant trimming (on the plane) as it's prone to leaning to one side (it doesn't favour one side, which one depends on wind and payload distribution). It's not a problem but at the same time I don't want to make it substantially more sensitive, something I would suspect can be an issue if I remove too much of the "corners" so to speak.

 

I've been wrapping my brain around the flat output curve of the engine and the future health of the transmission.

Perhaps you all have it figured out already but after some thinking and reading I realise that it doesn't matter what output the engine has at a certain rpm but rather what the prop allows for. So if I prop the boat properly I will get in this case 350 hp at 3000 rpm but less at lower rpm's.
Since I will be starting with the current prop and the same gearing I won't be putting more stress on the trans than today for any given rpm up to the current max of 2600 rpm.

 

I can follow that logic Magnus, so I'll think you're right.