Speed expectations for SIBs - past a 9.8

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decrepit

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We currently run a 9.8 Nissan/Tohatsu 2 stroke on a 12.5' SIB. On mildly choppy seas we hit 26 km/h (2 light people, probably 300 lbs, light load). Certainly a bit quicker on calm seas.

All is well with the boat and the motor - I love the weight of the Nissan. The boat has lifters, and with them planes with no effort. It all fits in my Nissan sedan.

It seems that SIBs are limited on top end speed because of their structure. Increasing HP to 15 doesn't seem worth it unless I had a planing problem. Increasing to 20 might... the question I have is what is the realistic increase I'd achieve with a larger engine? And would I notice, really? It seems people increase their HP to make their boats plane... the difference between 25 km/h and 35 km/h, experience wise, seems negligible.

As someone who hasn't gone past a 9.8 (but could take a 25, though need to pick it up, so wouldn't), I'm looking for opinions from people who have... do you "feel" the difference enough to justify the cost of a bigger engine?
 

gwozhog

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15hp yamaha 2stroke will be noticeably faster. 25hp would sceam but do you have the back for lifting a heavy motor. The 15hp 2 stroke is as good as it gets for small sib who do not trailer. Goodluck finding one though. Took me 6 months of combing the net.
 

Sea Rider

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Welcome to Iboats,

Extremely dangerous to overpower a light inflatable of that size with a 25 HP, usually rated for max 20 HP and max passenger capacity. What's the intended use. Boating alone speeding, boating with 1-2 up, carrying heavy loads ? If you're comfy witht the water performance of that 9.8 HP with 2 up, you can always maximize a prop pitch to take all the horses out of that OB and not buy a larger HP OB.

For that will need to install an induction tach, go for a wot spin you alone on flat calm, no wind water cond, check max achieved wot rpm and see if inside wot rpm factory range. Do same with 2 up and check wot rpm. For top Sib water performance will need to inflate sib's air chambers and keel to at least 3.0 PSI. Rigidity is all about regarding inflatable Sibs.

It's not all about higher top speed with larger OB's, but faster hole shot, carrying extra load, more boaters at a much better speed and overall water performance than with a underpowered 9.8 for that size Sib.

Happy Boating
 

decrepit

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I could imagine lugging anything larger than I have (Yamaha 15 2 stroke excluded, that seems reasonable to carry) would make me hate the larger outboard quickly.
I AM happy with the setup, and I'm hitting the average speeds a 9.8 can make (compared to what others with smaller SIBs state, I think we're going as fast as them). It comes down to balancing the budget and my desires.

Can I go faster? Sure! Will I get the speed I want by paying another 1000+? I don't know. For me, the only issue is top speed... the 9.8 has great hole shot. We suffered to get on plane with 3 (and 4 is just so slow I'm bored), but that was resolved with the lifters.

The boat is rated for a 25, 280 lbs max. Passenger weight is 6 people, 1273 LBS.

We live on one of the Gulf Islands (BC Canada), and the boat is used for island hopping and crabbing on occasion. Always in salt water, but never in the open ocean, we take it through the Pylades and Trincomali Channel. We usually have 2, but sometimes 3 in the boat.

I think I may be the only one to answer my question, haha. I'm just nervous giving up my reliable engine for something that may not be as reliable, and may not satisfy my "need for speed", especially when decent 2 strokes are hard to find. I don't want to race, but now that I'm comfortable in the boat, I can (and occasionally wish) I could go a bit faster. 9.8 to 15 still doesn't seem different enough unless the issue was getting it to plane with 3 people... a lot of comments I've read stated this as the reason people upgraded, not to get another 5/10 KM top speed.
 

decrepit

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Also, thanks for the prop pitch suggestion Sea Rider. We run a standard prop, it could be improved.
 

Sea Rider

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I like running underpowred OB's on large sibs/ribs propped right with tach, factory delivered prop is usually medium pitched, doesn't mean is correct for your set.up, that's if wanting to get full HP out of that OB. Invest on a Hardline hour/tach, tach that OB with 1- 2-3 up with current prop, will see how rpm drops damatically with each test. You can maximize a less pitch prop to rev around max rpm with 2-3 up. A larger Sib will perform much better with a underpowered OB than a less size Sib with same OB, slides more efficiently with less hull drag.

Happy Boating
 

gwozhog

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Buy the 25hp then and keep both engines. I own a 8hp and 15hp and would never sell either one. If you have alot of people going out you will have help to lift the 25hp. If its just you going out then take the 9.8hp. Its always good to have 2 engines just in case one is down. Nothing worse than sitting home on a beautiful day because you are waiting on parts.
 

Sea Rider

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Yep, that's why I have a (3) 5-18-30, last one is barely used. Buy a 25 if boating heavily passenger loaded and re prop the 9.8 for solo, + 2 up boating.

Happy Boating
 

gwozhog

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All two strokes Searider? Here are my two babies. I would love to add a 25hp yamaha and a 9.8 tohatsu 2 stroke. My 9.9 is actually a 15hp with new stickers because my boat is only rated for 10hp. Lol. You can never have to many outboards in life.
 

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alanfox55

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Here's my two babies. They are my two motors but I'm just kidding with you guys. :lol:

58.JPG

33.JPG
 

Chopperbill

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At 5000 feet the 9.8 Tohatsu (4 stroke) I had was a real dog. With two big guys it would hardly get to plane. I bought the Suzuki 20 EFI and it really woke up the boat. I only use it AZ (300 feet) now and it has a great hole shot and has gone 24 MPH. Rarley do wot and more as it planes real easy so we just cruise 15-18 MPH. The Suzuki 20 is the lightest in its class at around 95 lbs. The EFI is amazing. Rarely does it take more than one pull to fire up and immediately goes to idle, no choke to mess with.
 

JohnnyRudeClassics

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Seems like a good rule of thumb is to power the boat at about 70% of the maximum rating on the hull ID plate.
So for 25 HP, that would be about 18 HP. I have a 365 class China SIB, rated for 10 HP ( though I could have
sworn I saw an older version of the same boat rated for 15 HP, I think it may have to do with the heavier 4-strokes
we are now forced to buy in the states ). It runs well at wot in the 16 to 18 mph range with an old 1963 Evinrude
10 HP on the back. I also have a 1963 Evinrude 18 HP that I have tried and the rig just barely makes way at
about 20 mph ( on a 14 foot deep vee aluminum row boat, the 18 HP will make 23 to 25 mph ). I think if my prop
was in better shape, I'd say the 20 mph was entirely solid, possibly even push it up to 21 or 22 mph, but the fact
is the hull ID plate says 10 HP max, and throttling back to 14 or 16 mph is plenty enough speed for me.
BTW this is with 2 big guys on board and plenty of gear, with the entire rig, also including the motor and fuel,
weighing in at 850 to 900 lbs, if not 1,000 lbs at times.
 

Sea Rider

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Going back to original poster thread, a 15 HP will perform much nicer on a 12.5 footer than a 9.8 HP. If both engines are Tohatsu, the 9.8 is just 170 CC compared to 250 CC of the 15, 70 CC makes the boating fun difference. The issue with OP is that if currently sitting and removing the 9.8 each time the combo is used, doing same with a 15 heavier engine will be a real PITA and a killer with a 25. You could always maximize one less prop pitch to pull revs up, better hole shot, throttle less once on plane and have goof fuel consumption powering that 12.5 footer which is rated for min 15 and max 20 HP engines.

Happy Boating
 

JohnnyRudeClassics

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A 365 class SIB is very close to 12 foot, kind of close to 12.5 foot.
Just thought I'd post some results with my experience with a similar craft.
The 10 HP is 16 cu in (262 cc) and the 18 HP is 22 cu in (360 cc)
( boy oh boy, it is amazing what schnuerle porting (SeaRider's Tohatsu)
did for outboard performance and fuel economy
versus the older cross flow designs (my archaic Evinrudes) ).
And you are entirely right, the larger displacement provides more stump pulling power off of the starting line and will move a larger load with ease.
This is true even when comparing my 18 HP to a more modern 15 HP Mercury of mid 90's vintage ( schnuerle porting / 262 cc ).
Off the line, that old 18 HP Evinrude just plain old has a feeling of more command ( 360 cc vs 262 cc ).
On the top end of the RPM curve, I think the 15 HP Mercury actually runs a bit faster with moderate load.
The Mercury definitely gets way better fuel economy, have not measured it, but it seems almost, if not at least, double.
 

ExplorGM

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We have a Saturn HD385 (13ft Saturn with center console seating,steering wheel 12volt battery,6.6gallon tank and aluminum floor) and a brand new Saturn ZK430KaBoat (15ft) with nothing but a 6gallon tank and an air floor. Probably 200lbs lighter than the HD385.

Engines are 2006 5hp Honda a 2011 20hp Mercury and a 1975 135hp Chrysler...

With the 20hp the HD385 does a solo up top speed of 20mph/32kmh. The ZK430 did 23mph/37kmh this weekend on our first trial run. With 4 people onboard the speeds are 12mph and 20mph repectively, with the 20hp. Inflation and weight is everything with these two boats. With an underinflated keel the HD385 could barely do 7mph with 4 on board.

The longer narrower ZK is lighter, faster and more manageable with 4 on board although at 23mph with just a driver it gets a little uncontrollable as it will get air under it and buck to one side or another without warning... 20mph is just about right for solo cruising. the ZK probably has more speed in it with some trim and prop work. It does have a Stingray Hydrofoil but cavitates and loses traction occasionally in wake situations on the ZK. The 5hp is light to carry but too slow to bring. It has now been relegated to kicker use on the sailboat. The 20hp is heavy but with two of us manageable and definitely worth the effort!
 
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