Twin Jet Boat Build

[Sms986]

I haven't forgotten about you guys. I'll keep you updated when new stuff comes up. For now it's just a lot of waiting.

 

[DogCavalry]

2 things from over here in madness:
First, as an air guy, I assume you're familiar with low drag inlets used in high speed aircraft. It seems they'd make a meaningful contribution to a jet's efficiency. After all, it's about delta v times m. Not enough m, and you're reduced to too much delta v², and we all know how that goes.
Second: hijack moment. I feel your pain on the ice front. I invented a new kind, but didn't pursue it because change is in the wind. So I invented a new kind of wind turbine, and various sub components. Alberta government offered me a quarter million bucks of oil money to develop it, for greenwashing purposes. Then the price of oil collapsed, and that was that. Heck, it's no wonder I have the madness in my brain.

 

[DogCavalry]

Speaking of streamlines...

Wouldn't they kind of shed from the center of the hull out in a v? That's my main thing with a vee hull and two jets.
With a strakes flat bottom it's pretty straightforward-channel/BL flow straight out the back in between the strakes. The vee hull is a bit more complicated in the way it sheds water. Much more dependant on a lot of other things.

May I suggest an inverted V? No particular bias on my part...

 

[Sms986]

...I think a twin vee is possibly above the capabilities of the fab shop, and certainly above my budget, lol.

I have been thinking of some interesting jet boat stuff, such as a y shaped intake with grates vertically mounted on the inner sides of a jet tunnel hull, wrapping around the driveshaft instead of up to it. No more negative deltaPE.

I'll have to look into the low drag inlets. I am a very novice air guy-always wanted to be an engine guy, but thanks to *cough cough*, the IC engine department in the automotive industry is nearing an end.

I'm literally still trying to wrap my head around why a strong adverse pressure gradient after the separation point is still bad (thesis stuff)

 

[DogCavalry]

Thesis point: now I'm wondering too.

 

[Barry]

...I think a twin vee is possibly above the capabilities of the fab shop, and certainly above my budget, lol.

I have been thinking of some interesting jet boat stuff, such as a y shaped intake with grates vertically mounted on the inner sides of a jet tunnel hull, wrapping around the driveshaft instead of up to it. No more negative deltaPE.

I'll have to look into the low drag inlets. I am a very novice air guy-always wanted to be an engine guy, but thanks to *cough cough*, the IC engine department in the automotive industry is nearing an end.

I'm literally still trying to wrap my head around why a strong adverse pressure gradient after the separation point is still bad (thesis stuff)

BRP has been making twins with the smaller jets and a thorough check on the internet shows other aluminum boat manufacturers using twin Sportjets, certainly one of the main suppliers to other OEM's
Scarab is using the BRP in twin installations as well. A lot of engineering has been done on these. Perhaps a visit to a few jet boat forums will give you some more information about twins, orientation etc

 

[Sms986]

I mean think about it. Consider a streamlined piece of airfoil (the sideways teardrop). The form drag is a summation of pressure forces along the body. I call the fore portion the leading edge to the thickest cross section- then the aft section is the downward slope from here to the trailing edge. form drag on the fore section is a high pressure acting to push the airfoil backwards. as the geometry relaxes, the high pressure reduces (a favorable pressure gradient), then boop, it separates. So by definition, an adverse pressure gradient is the increase in dynamic pressure as a function of downstream distance. By nature, there exists a low pressure zone on the aft section of the airfoil, and negative pressure acts to "suck" the airfoil backwards. So in contrast, positive pressure in the aft section would act to push the airfoil forward. Therefore, if the pressure gradient is wildly adverse after the point of separation, how does this not help the pressure become positive faster and therefore reduce overall drag? Lol sorry.

Barry, I have checked out the Yamaha twin jet boat, but I haven't found much info on it. I'd love to see the pumps from the inside of the boat.

 

[DogCavalry]

I see your point. I'll bust out my old copy of Marchaj.

 

[DogCavalry]

Check out my thread of an inverted V build:
Sea Sled Madness

 

[Sms986]

That's really cool! and probably expensive.

Here is my current project (almost complete)
1987 Larson DC-215 restoration https://forums.iboats.com/threads/1987-larson-dc-215-restoration.716568/

I know it's on another forum...

 

[DogCavalry]

Sms986, I'm well into your restoration thread. Thanks.

I don't know if my boat build is expensive or not. I'm at about 7500usd for materials. Maybe another 1000usd for materials to complete the hull. After that it gets vague. A pair of good used outboards. Electronics. Safety gear and rigging. Not free, but about a fifth the cost of buying the boat.

 

[Sms986]

I'm about $10k into mine, but I'm just doing a gut and restore, not building an entire boat like you are. You can also see how novice I was/am with fiberglass boatbuilding lol.

 

[DogCavalry]

The biggest costs in mine are epoxy, outboards, electronics. And of course about a thousand hours of my labor.

 

[BlueBell]

Therefore, if the pressure gradient is wildly adverse after the point of separation, how does this not help the pressure become positive faster and therefore reduce overall drag? Lol sorry.

You're familiar with the Carmichael bulb experiments?

 

[DogCavalry]

Not I.