nzjetboating Web Board. Forums => Projects => Topic started by: P.B. on May 25, 2021, 16:58:05 PM

Title: mid size boat engine and unit options
Post by: P.B. on May 25, 2021, 16:58:05 PM
Hi, I am looking at commissioning a new build and would like to get your thoughts on a few ideas before I commit to the build. I find I have learned a lot by scrolling the forums and I appreciated the sharing of knowledge and the time spent contributing by the jetboat community and people that know far more than I ever will.

I currently have a 5m boat with a 350 Kodiak and 212. It has been a great set-up and used for a lot of time on the lake and waterskiing along with some river work. It was bought with the intention to have one boat to do everything - young family boating, lakes, water sports and the rivers (predominantly, the Waimak both lower and gorge, Wiklin, Matukituki, the Dart and some others along the way). As highlighted here, one boat for all is a compromise in most departments.

The issue I find in the large heavy jet boat is on the braids you are constantly boating in a state of if I stop I will never get going again and with a young family, this is not ideal. The boat is heavy to push 1050kg + (yes I have a winch but still a lot of work). Every time I stop in what I think is reasonable depth water I can not get going again without a lot effort and few stones pumped, when many lighter v8 and other boats seem to have no issue.

I have made the call to have two boats going forward, I will have a dedicated lake / watersports boat and a river focused jet boat. Now for the jet boat part, I think I am leaning towards a 4.2m size hull. I want something that is still big enough to take the family out or 4 adults on a boys trip and some gear. I do not want to go down the jet ski type size, but equally, there is no point in getting a 4.6m + boat as it will be getting close to what I have.

I am after a boat and set up that meets the following objectives:

- Reliable and low maintenance (for a jet boat anyway)
- A lot lighter and more braided river friendly than my current boat
- Take 4 full-size adults and some gear if needed
- Can get started in shallower water to avoid boating in fear when in reasonable but shallow flow
- Reasonably fast, responsive and quick to accelerate even with a load
- no ear muffs being required
- safe for young kids
- can pump gravel without needing to be rebuilt every outing
- can handle some white water use and the odd clip of a rock without folding like a can

Yes, I know I am looking for a unicorn so to speak but I am trying to build a long term boat that serves the above uses to 95%, 95% of the time, and the other 5% is the compromise. The budget is reasonable to do it once and do it right.

I am keen to hear your thoughts on:

- Is a circa 4.2m size boat the size I should be looking at for the main rivers I boat and how I want to use it?

- What is the best engine option for a boat of this size with weight in mind. I do like the idea of sticking with a v8 but have been told it may be too heavy in the back of a 4.2m type size hull and having to have a suitable unit to match the v8. This then wouldn't meet the criteria of getting started in shallower type water? I guess I will also be giving up leg space in the back seat. What is the smallest and lightest v8 that would be suitable on a boat this size if it was a possibility?

- If I gave up the sound of a v8 for a v6 is the VQ35 still the way to go? How does the new DI VVT 4.3l v6 stack up in comparison? How much performance do I lose going to a v6 compared to an ls type v8?

- If a v6 is the answer, what is the best pump option for performance, durability and reliability? Scott 812, Flow pro, Southern jet SJ216 SINGLE STAGE any others I should be looking at?

- Best hull construction for all-round boating? Do you go for Raex steel plate, plastic or say a 12mm keel?

Thank you for taking the time to read the wish list and I appreciate any feedback, views, own experiences and expertise on any of the above.



p.s. if you have the unicorn boat above and are thinking of selling ill be keen to look at that too.

Title: Re: mid size boat engine and unit options
Post by: Ross Goldsack - JBNZ Immediate Past President on May 25, 2021, 22:47:26 PM
Where do I start?

I think you are correct and it's the jet unit that's the hardest part of the puzzle to get sorted.

The next critical thing is weight, a 4.2 with Raex and a heavy engine and big unit may take as much water to get going as your big boat. Be very careful about adding weight.

For the Rivers you mention I wouldn't go Raex, HDPE plastic would be a better option IMO (my big tub has Biz and it sure is tough but heavy)

A 4.2 is a small big boat... a 3.9 is a big wee boat (sort of)

I'm lucky enough to have a few boats, big, medium and small. The medium one does most of what you are after, carries 4 adults, doesn?t draw too much water at rest and does 95kph. Jettec 385 (about 4mt long) Honda K24 running up to 20psi boost and a SJ 7 inch 2 stage. It goes pretty good but the unit is a pain in the bum, has to be over pitched or it slips like crazy. It's the boat I take for adventurous family boating.

I sometimes get a bit excited about building a 4.l with LS1 and 2 stage Flowpro. Keep it light and basic and it should be a weapon..... or LS 3 and bigger unit....and so it goes on
Title: Re: mid size boat engine and unit options
Post by: Jeff B on May 26, 2021, 05:58:22 AM
Your quest for the perfect boat is neverending... l-l

Too many times someone sets off to build a nice, simple, lightweight boat and by the time all the extras are added it's a shiny, overweight, expensive garage ornament.
Like Ross has mentioned, a 4.2m hull with decent v8, decent unit and 4x adults will need same size pool to get going as your current boat (if you do the impeller up)
Put a v6 or even 4 cylinder will draw a bit less but need longer pool to get up on plane.
Jetski running gear even lighter again but after pumping a few stones on the way upstream you will have to throw out a few passengers for the return trip as it travels onto plane...
As I see it,  4.2m hull will really be no different to the common 4.4,4.5m hulls common?
Go down to 3.9, 4m and it will be a rocket with alloy v8 and still pretty lively with v6?
Title: Re: mid size boat engine and unit options
Post by: steelo on May 26, 2021, 15:50:22 PM
Pb. I?m looking for the unicorn boat. Can it be done? Budget no object
Jeffb. Hold my beer.
Title: Re: mid size boat engine and unit options
Post by: P.B. on May 26, 2021, 19:41:22 PM
Ross, thank you for the reply. Yes, 3 boats will be the logical answer to solve all problems! The CFO at home might struggle with that one. I am going for the two good ones approach to keep the family happy and to cater to all our adventures and activities we enjoy.

How do you find running a turbo from a heat, reliability and longevity point of view? I am not too mechanically minded, so the less issues I have the better. I try to be preventative in servicing etc rather than reactive with my toys. I did think if the VQ35 turbo as a good substitute for the v8 but might get a bit too complex for my limited knowledge o engines.

The 4.1 ish boat you mention with a ls1 was my original thinking, until I was advised it might still be too heavy in bum to shallow start. What has stopped you from pursuing something like this as a set-up? What do you think would be the downside of an ls1 in a 4.1m boat and is there a reason why you would favour the 2stage flo pro for this kind of set-up?

Thanks Jeff for your thoughts. Yes, the quest is never-ending, but I was hoping with modern technological advancements and a reasonable budget I could at least try! From what I have been told, a 4.2m can be built a bit lighter and is narrower at the chine and with a v6 could be 650kg all up compared to 900kg a 4.4 to a 4.6m boat. I find my 5m boat (Vulcan) doesn't seem to have a lot of lift in the rear to get going compared to say a 5m kwikkraft. Yes it has a lot of creature comforts, but hence why I am looking to move to two boats to reduce the need of the extras on the river boat.

Thanks Steelo, make sure you find two unicorns while you are at it!



Title: Re: mid size boat engine and unit options
Post by: Ross Goldsack - JBNZ Immediate Past President on May 26, 2021, 20:05:05 PM
The turbo hasn't given me any grief but I've only got 90 hours on it. Turbo is water jacketed so heat isn't a problem. The biggest downside is clutter and complexity in the engine bay plus some turbo lag.

No reason I haven't built a 4.1 LS yet other than time/$$$/shed space. Might be my next project. Ive never owned a Flowpro but they are beautifully built and a 7.5 inch 2 stage works pretty good with a stock LS1.
Title: Re: mid size boat engine and unit options
Post by: explorer on May 26, 2021, 20:54:52 PM
Hi just got going my Hot Rod  its 730kg with a 100 litres fuel
The hull is a 3.9 scott with 8.5 unit ,powered by ct525 chev , we changed the cam to get more low down grunt
I built it with know extras like carpet to keep it as light as possable , but it is the ultimate fun boat to drive
Title: Re: mid size boat engine and unit options
Post by: tyro on May 27, 2021, 13:08:42 PM
I have been enjoying this guy's video's the boat is a 4.1 westlander with vq35 and Scott 8 inch unit  It looks like it can do what you are thinking of?
Title: Re: mid size boat engine and unit options
Post by: Warwick Marflitt on May 28, 2021, 17:48:51 PM
So am I right in thinking that a bigger jet unit requires a larger volume of water under the boat for it to load the impellers. Wouldn't less dead-rise give you a better hull buoyancy displacement ratio? Realizing that jet unit needs to be low enough in the water to pump....?Find & Follow the Unicorn poop..X+Y/Z=B-L sqd^ to the power of the Thrust divisor AND ITS % of $$$$$$ equals GOLDEN POOP......If you want too find A UniHorn  B))T B))T B))T       
Title: Re: mid size boat engine and unit options
Post by: P.B. on May 28, 2021, 19:51:31 PM
From my limited knowledge I am lead to understand the 212s do suck down a bit. Great for thrust in deeper water - I have towed a 6m sinking boat full of water on the lake up on to the plane with it, but not so good for getting going with a load in the shallows.

I have wondered if a tilt nozzle on the 212 will assist in pushing the bum up on take off in the shallows,  while countering against the intake sucking down.

However having 17 degrees dead rise prob doesn?t help with getting going with a load in the shallows too
Title: Re: mid size boat engine and unit options
Post by: P.B. on May 28, 2021, 19:56:31 PM
Is the flo pro 2 stage the preferred unit for a standard vq35? How do you think this compares to the southern jet 8.5 inch single stage? What would be the main advantages and disadvantages?
Title: Re: mid size boat engine and unit options
Post by: swifty on May 28, 2021, 22:30:31 PM
Tell us what hull you have Mr p.b .Pictures are good too.
Title: Re: mid size boat engine and unit options
Post by: P.B. on May 28, 2021, 23:59:22 PM
Hi Swifty - I have the Hartz Vulcan at present. 212 kodiak 350. All standard but boat is fully optioned so quite heavy.

Thinking of moving to the 420 Jettec / rapid runner or similar size with the vq35
Title: Re: mid size boat engine and unit options
Post by: ntimz on May 31, 2021, 20:31:52 PM
+1 for the two stage flo pro and ls1.

I had this exact setup in a 4m LVB hull which ended up being fractionally too much running gear for the hull size!

In hind sight I would have gone with a 4.2, would give you genuine 4 seats, 100kph+ and just all out great fun to drive!
The southern jet 210 pump goes great on the LS1, and prefer this pump on bigger (4.6) setups, problem is it?s a lot chunkier
Than the flo pro, and it all starts to add up in a small package hanging out the back. I have heard great things about the southern single stage, although no direct experience...

From what I understand the 212 was never intended to be a shallow water pleasure boat pump, and as you say they have their limitations in shallow braided water (especially getting going) Don?t get me wrong they have thrust to burn, but I have seen too many mid sized river boats sucked to the bottom trying to get going where others have no issues at all....

From what you have said, this combo would meet your criteria, and probably a v6 Nissan/Honda etc would be close behind  2c
Title: Re: mid size boat engine and unit options
Post by: JimJet on June 18, 2021, 20:17:43 PM
Hi PB. Like the others said options are infinite and every boat is a compromise. Over the years I've owned quite a number of boats and every one without exception was just what I needed when bought it or built it.
My latest "perfect" boat is a 3.6 metre alloy boat, 14 degree, plastic keel, 3.0 Honda, Berkely pump. 600kg all up including some fuel. Thats with a massive heat exchanger (had it under the bench).
If you want a river boat personally I wouldnt go more than 14 degrees. For me 80kph is any amount of speed if you dont want to frighten the bejeezus out of the wife and kids. You dont want to be boating on your own. Well maybe you do. There are plenty of good V6's 3.5 to 4.0 litre. Mainly Nissan or Honda. Hondas have a single port exhaust so cheaper manifolds or easier to make your own.
Fuel tank either under the rear seat or even under the front seat to keep the weight more balanced so easier to the plane.
So many good pumps out there now. Just match the power/weight/performance that you want. For me the Berkely/American Turbine's are bullet proof for lack of maintainance. They have their limitations but everyone has their own preference.
Im already thinking about my next "perfect boat"
Title: Re: mid size boat engine and unit options
Post by: JimJet on June 18, 2021, 20:27:34 PM
Had to resize photo
Title: Re: mid size boat engine and unit options
Post by: Warwick Marflitt on June 18, 2021, 20:35:06 PM
As the jet unit lifts the water up into itself the weight of the water will lower the hull into the water. So if jet units internal volume is say 40 liters then you've got 40kg of weight moving from river/lake up into unit increasing hull weight.(Newtons Law) Get two buckets of water and lift them off the ground at speed of boat jumping onto acceleration, to give you a hands on idea? Don't stuff your back!!!   
Title: Re: mid size boat engine and unit options
Post by: JimJet on June 18, 2021, 21:11:26 PM
Forgot to mention.
 My 3.6 with the 3.0 Honda does 12 litres per hour on hour metre. You would have to go direct injection to beat that. I've set the revs to a low 4300rpm so I am only pulling about 160/170hp at those revs rather than 230hp at 6200. Much quieter. The weight of all those internals remains the same (engine, seats, fuel tanks etc) so if you want your boat to have less draft then just add a bit more length/width. It will carry more load, be easier onto the plane, will take off in shallower water but will have a bit more wetted area when planing so a wee bit slower.
My boat does everything you were looking in your 4.2 except the V8 noise and the 100kph.
Title: Re: mid size boat engine and unit options
Post by: silvafox on August 01, 2021, 13:24:11 PM
Must be time for another one 😉