NW Jet Sled Build

Hi,
Long term lurker here. Like many, lifelong boat and fishing nut located up in Washington state. Have had half dozen jet sleds, couple drift boats over the years and regularly captain buddies offshore boats. Over the years, I've driven a lot of sleds, talked to a lot of builders, fished many different models and done a lot of research on here and other sites. From taking small aspects and characteristics from several boats, have decided to try and come up with my own design. Recently purchased Rhino/Orca for another project and so it would be designed in that. Hoping to pressure test the design here and get feedback. The amount of knowledge and willingness to teach on the forum is pretty amazing.

Goals:
-Shallow water, outboard jet tiller driven sled that can float in 5-6" of water and run through 2-3" regularly with 4 people. Very quick to plane in shallow water and very maneuverable in tighter gravel strewn rivers. Mainly used in gravel bottom rivers without shale/bedrock type structure.
-Tracks good and fishes great. Meaning, boat floats flat at rest, tracks well for drifting and is consistent. One aspect that is important is that boat can pivot, so to speak, on the middle of the wetted length while under power with a 8hp or electric kicker motor at the stern. For the style of fishing we do that makes the boat very maneuverable in tight spots with little motor input.

Design
-20' overall length x 78" chine width. 25-26" sides, overall beam TBD. (small as possible to fish 4-5 people, avoid floatation weight and balance a 175-200hp, 430-450# outboard)
-.190 or .160" bottom, .125" sides with formed rib. Additional .190" or more doubler welded on inside on pad area and at bottom corners of tunnel for impact reinforcement.
-Very long planing surface (horizontal keel, still learning official terms). Around 17'. (maintain shallow draft and consistent tracking)
-4° deadrise in transom. Constant throughout planing surface but possible move to no more than 6° at bow if needed based on shape. (4°-5° deadrise seems to be the optimal deadrise for tracking the way I fish. Deeper v are slow to react when fishing and flat bottom are too loose)
-4" wide chine flat set to horizontal to reduce draft. Chine flat maintain constant width for entire wetted length of hull. Unsure on extrusion to join side/bottom or weld and cap and incorporate outmost lifting strake into the cap.
-16" wide flat pad on keel that maintains for entire wetted length.
-16" wide flat flows into a shallow tunnel. 16" wide at entrance, tapering to 14" wide at the roof at the stern. No more than 15° angle on roof. Tunnel depth is only 1.75-2" deep with 60° walls. Depth is shallower than typical hull Tunnel extends 6" past transom. Tunnel roof will be continuous with hull sheet to make sure smooth waterflow.
-16°-17° transom. More angle so that when the jet is trimmed out there is some setback. This has allowed me to raise the motor slightly on non-tunneled hulls before and hence the reason for the shallower tunnel on this design.
-3 lifting strakes each side. Inner 2 will be 5° down from horizontal. Outermost lifting strake will be on the chine flat and extend to chine itself possibly more downturn. Used to enhance steering and reinforcement for impact.
-sides will flare out aggressively from chine to maybe 4-5" vertical then the sides will go more horizontal to hit gunnel beam. (increase floatation at rest, provide more interior room for lower rod tray and reinforcement for side impact)
-Bow shape will be very abrupt to maintain long planing surface. Still learning technical terms but I want the bow broad and wide so the bow will float over rapids rather than dig. Bow will not come to a single point but rather a modified flat/pointed shape. Trying to keep surface area wide just above waterline for when people are standing on the bow or when going through large waves.

Would greatly appreciate any feedback or suggestions. I'm a big theory/research person so getting the shapes to form the way I want is still something I'm learning. I have several books on aluminum boat building and am doing my homework.

Thanks
Sean

 

I will just embed some comments within your post
Many of the boat builders in western Canada are adding a layer of UHMW to the hull to be able to slide over rocks. While it does add weight, you do gain additional impact resistance as the uhmw spreads out the impact zone. If you are trying to run in 5 inches of water, it may be something to consider. You could then go back to a .188 inch/.190 bottom thickness.
Re tunnel ventilation. River Road Marine makes an addition to an outboard jet that keeps the engine direction constant but turns just the nozzle. Ie no tunnel ventilation, I have tried to get in touch with them but they have not bothered on several emails and a couple of phone calls.

I don't see the advantage of a tunnel. The other weakness is if you slide over a sharp rock, the longer extension might hang up on a rock and break. I would make some improvements here. Also the back of there tunnel seems to turn down to match the intake of the pump, I think it should be built parallel to the level of the keel.




1760 https://www.riverroadjetboats.com/1760.html This is the second link, note that they use the uhmw and can bump over the rocks.

The third link

 

Barry,
Thanks so much for your input. I was hoping this would perk your interest.

I have seen the River Road nozzle system. I think it is fantastic. however, i'm looking to build a tiller steering boat and I'm not sure how to rig a tiller/cable steering system. UHMW is nice, but it tends to make boats track funny in my limited experience. Similar to putting wetlander or another bottom coating on a sled.

Guess i'll expand a little on what I'm calling tracking. I'm designing this sled for free drifting and backtrolling. Not quite sure if you're familiar but free drifting is where the boat is drifting current speed and we're dragging/drifting baits off the side of the boat. Shallow draft and a low deadrise helps keep the boat drifting along without being caught in boils, seams, eddies, etc. The absolute best free drifting hull is a 4 degree with a wide pad on the keel and that's the base of the design. Something about this combination just makes the boat drift perfect and very minor adjustments to slide the boat laterally are very easy. One would think a flat bottom would be best for this, but the very slight longitudinal stability of a 4° with lifting strakes just does the trick. For backtrolling, this hull is nearly perfect, you just have to have people slide their chairs towards the stern to plant the stern a little deeper.

I have this sled in a 17'x66" right now. It doesn't have a tunnel and if it weren't for the urethane shoe I'd be in trouble a lot. One thing I did to my current sled, which make a drastic difference, was add a 2° wedge on the transom to bring the overall transom angle to a hair under 17°. This angle, combined with nearly perfect weight distribution allows me to stay on plane down to around 13 mph. This is perfect for negotiating very tight turns and small rivers. Usually when the rivers are that tight, they are a bit deeper so trimming down and going slow is acceptable. Then, we I have a very shallow, long riffle to cross, due to the transom angle and neutral weight, I can trim the jet up until only 1.75" or so of the intake actually hangs below the boat. It seems as those by trimming out so far, I'm creating setback for the jet intake. Similar to prop applications, I was able to raise my jet 1 hole to where the front lip of the intake is now 1/4" above the bottom and I get virtually no cavitation. My idea for this sled, is to design the tunnel just for those times where I need to cross shallow straight riffles. If the tunnel is only 1.5-1.75" deep with flatter walls, my thought is that I wouldn't experience any ventilation. When going slow I could still trim down completely and the shoe would be well below the tunnel, then when I trim out all the way for those riffles it would be nearly flush with the bottom of the hull.

With regard to strake angle, my current hull utilizes strakes that have around 12° downturn from horizontal. They seem to provide good lateral stability as the hull really doesn't slide much. Given the strakes are just for steering and don't provide much lift, is there an optimal short side angle you've found for steering? With 12° downturn (horizontal), that would put the short side at 78° above horizontal if it's a right angle. Seems as those that may keep the hull flatter in a hard turn as it would provide lift in a slide. The chine flats aren't as much for lift, but rather just to reduce draft. I would plan on putting a strake on the chine itself to cap it and probably stay with 12° downturn as well.

 

I was also curious on the tunnel roof why taper would possibly be negative? I assumed a slight bellmouth, ie wider entrance at the fwd edge, slightly narrowing may help keep the tunnel pressurized a little better. Thinking speeding the water up might be a good thing from a couple standpoints.

 

Mitch184: I think I must have missed something throughout the discussion, so please forgive me. What hull are you looking to modify? Is it your own design, or is it another’s? Either way, the hydrodynamics simply cannot be discussed without all of the information.

 

Therandone, first post has all the information. This is a design I came up with. Its not modifying an existing hull. New hull.