Bo Jest: power questions, engine type

Designs for inboard or outboard power

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scout
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Bo Jest: power questions, engine type

Post by scout » Mon Apr 29, 2013 6:12 pm

hello all.

i'm in the planning stage of a Bo Jest. i've gone through all i could find on the forums and have found lots of great info.

i have a question concerning engine choice for the boat. i have noticed most have been built with an outboard engine. any advice/opinions on using an inboard?

also - most of our cruising will be done on lake michigan, and in a post i read regarding current/power i am now hesitant about the cruise speed of 6 knots being enough. we don't mind going slow, we just want to have enough boat to handle the waters :)

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Re: Bo Jest: power questions, engine type

Post by Trackhappy » Mon Apr 29, 2013 7:52 pm

Hiya Scout,

I LOVE inboard diesel engines and if I built a tug type boat it would take a lot of convincing for me to use anything but twin inboards diesels for maneouverability and redundancy, however......
You lose valuable internal space.
You add the complication of a shaft and ancilliaries
It is more complicated and will take longer to build
Less choice in cheap easily available engines
If you have problems, and outboard is easy to change out.
Noisier running
Issues with fumes, heat etc inboard

If like me you love inboards, go that way. If you want a useable, care free boat with more internal space, go outboard. Alternatively, go out and have a Voith Schneider drive system built in small scale (OK, just threw that is as a way out there dream of mine :oops: )
By the time I have built a boat, I'll be ready to build a boat....

scout
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Re: Bo Jest: power questions, engine type

Post by scout » Tue Apr 30, 2013 11:11 am

thanks for the reply :)

i'm going to go with an outboard -- saving space and $. i want to be able to extend the pilot house and we want every inch of livable space we can squeeze out.

as to the 6-knot question -- we've decide that the lake michigan waters, even close to shore, aren't going to be for us. the chain o lakes (we'd put in at Fox Lake) here in Illinois have always been fun for summer cruising and we can trailer to other lakes we want to explore as well.

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tsmitherman
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Re: Bo Jest: power questions, engine type

Post by tsmitherman » Thu May 02, 2013 9:47 pm

Scout,
I have almost 2000 miles on my Bo Jest, and I've been very happy with the 9.9 Yamaha High Thrust model outboard.

When I was trying to decide on an engine, I talked with a lot of people, and 95% of them told me to get a larger engine. Their reasoning was that a larger engine was safer if you had to outrun a storm, better when going against the current, etc.

But the one person I met who made me decide to go with the 9.9 was Tom Hardison, who had built a 24' Noyo Trawler and had travelled extensively in it on the Tennessee River and Tenn-Tom Waterway. The Noyo Trawler is a much larger boat than the Bo Jest, and Tom's enthusiasm for the 9.9 convinced me that it would be plenty big enough for the Bo Jest.

I typically travel at about 5.5 - 6 mph, at about 3800-4000 rpm, and burning 1/2 gallon of gas per hour.
That still leaves just enough throttle to push a little harder through the current or wind if necessary.
Tom
------------------------------
36' 1969 Willard Aft-Pilothouse Trawler
Blog: www.genesisboat.blogspot.com


Knot-So-Fast (BoJest) SOLD

"It's amazing what one can do when one doesn't know what one can't do." - Garfield

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raymacke
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Re: Bo Jest: power questions, engine type

Post by raymacke » Fri May 03, 2013 7:16 pm

Tom and I are both card carrying members of the "Slow and Steady" club. We travel rivers with current but still are very comfortable operating at displacement speeds. The Bo Jest is a semi displacement hull (as is my True Grit) which means it can be pushed beyond the hulls theoretical displacement speed* of approximately 5.5 kts or 4.8 mph. That is why Tom can run at about 6 mph with very reasonable economy. Trying to push a true displacement hull of the same length to that speed would require much more HP and fuel and is usually regarded as folly. Plus usually the displacement hull won't plane unless the HP is extreme and maybe not even then. A semi-displacement can be pushed to plane but will be VERY inefficient when doing so - at least when compared to a planing hull. A planing hull rides on top of the water while a displacement (and to a big extent semi-displacement hull) pushes the water out of the way. To me the semi - displacement hull is a compromise to allow a little faster speeds on "special occasions" and really is not intended for extended faster cruising.

I guess my point is if you want to cruise faster than 6 MPH The Bo Jest may not be the best choice of designs OR at least be prepared for far more operational cost with relatively little true increase in speed. I have a fuel flow gauge on my True Grit (displacement speed 6.7 kts or 5.8 mph) and it shows at 6.5 mph I am burning 1 gph. If I push it up to about 8.5 mph my fuel burn nearly doubles. At 11.5 mph I am burning 5.4 gph! So to gain 5 mph in speed I am burning more than 5 times the fuel. And still, I am only going 11.5 mph which sure won't blow your hat off. Also, I will admit if I slow it down from 6.5 to about 6 mph, which is close to the theoretical displacement speed I save about 25% but burning a gallon per hour makes the math easy so I so I pour on the coal and blaze through with that extra 1/2 mph. :mrgreen:

Another way to look at this - a rough figure for gasoline engines are they burn .1 gallon per hour per horsepower. So to cruise at 6.5 mph I am using about 10 hp. To cruise 11.5 mph I need 60 hp. Again, these are rough numbers but you get the idea - it takes a lot more horsepower to go a little faster in this type of hull. So in my opinion you can successfully run the Bo Jest at higher speeds but you need to be aware of the downside, which is far less efficient operation.

Also, Tom and I both decided on the "high thrust" series of outboards produced by several different manufacturers. These spin larger diameter props at lower rpms and are ideal for slower moving boats. Yamaha, Mercury and Honda all have these available.

-----------------

* In case you are not familiar with the numbers the theoretical displacement speed of a displacement hull is usually calculated by finding the square root of the LWL (length at the water line) and multiplying by 1.34. This yields the speed in knots. OR the Bo Jest LWL is 17' and the square root would be 4.123 times 1.34 = 5.52 kts.
So Many Rivers,
So Little Time....

slug
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Re: Bo Jest: power questions, engine type

Post by slug » Sat May 04, 2013 2:51 am

Scott;
Basically what Tom and Ray are telling you, is that you are making a choice for "cruising style".
My wife and I have been on both sides of the spectrum, with runabouts and sailboats, and now the Titan tug. We will tell you the "slow speed" approach is our preferred choice. Much more relaxing and scenic. Time to look around at the wildlife that few others even see, while enjoying a coffee or lunch on the go at your leisure (still moving mind you!).
Both of their postings are well thought out, very informative and based on excellent personal experience.
You won't get a better answer to your question.

Doug

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Re: Bo Jest: power questions, engine type

Post by scout » Sat May 04, 2013 7:41 am

all the info is greatly appreciated :)

i do understand the limitations of the hull on bo jest. i was just wondering if i should allow for extra hp for use in waters with a bit more current/wind/chop to deal with.

was thinking about the 12 hp but not trying to go faster -- just looking to know i have enough under conditions that require it.

but, as tom said -- he has a bit extra for those situations, even with the 9.9.

i'm at an age where getting up on plane and burning fuel like its cheaper than water is not appealing LOL. slow and steady suit me just fine.

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kens
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Re: Bo Jest: power questions, engine type

Post by kens » Sat May 04, 2013 5:31 pm

If I may chime in,
I hear about the same story as well, I am the offshore fishing boat type (not card carting slow & steady). All the other guys say you need extra big HP to run out of a storm, chop, or whatever.
I'm throwing the BS flag at all that.
The displacement guys aint outrunning a storm at 8kts, and for that matter neither is a offshore center console at 30kts. The only reasonable way to outrun a storm is to head for home before it gets to you.
I have been caught in bad weather in a sterndrive boat offshore and the salt spray blew aft to the transom (as it always does) and started cross-firing the spark plugs. This was scary situation as I needed power to maintain steering (and get home) and power was iffy at best.
My point is that HP don't matter at all. What you need is horsepower that is dependable in all weather conditions, and sea conditions.

10HP that runs thru a sea storm is better than 300hp that fails you......................
Besides that, when the weather kicks up you actually slow down anyway because you cannot run fast in rough choppy seas.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Re: Bo Jest: power questions, engine type

Post by scout » Sat May 04, 2013 8:23 pm

kens wrote:
My point is that HP don't matter at all. What you need is horsepower that is dependable in all weather conditions, and sea conditions.

10HP that runs thru a sea storm is better than 300hp that fails you......................
Besides that, when the weather kicks up you actually slow down anyway because you cannot run fast in rough choppy seas.

agree 100%

i think i'm going to end up with the Yamaha 9.9 -- seems tried and true and tested by the other bo jesters :)

scout
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Re: Bo Jest: power questions, engine type

Post by scout » Sun May 05, 2013 12:17 pm

and yet another question :)

in reading through the yamaha site, and the owners manual for the 9.9 it says

alternator output 6A (requires rectifier/regulator)

then on page 13 of the manual it basically says this is NOT a good idea.

what are you guys doing to charge your batteries? i'll have shore power that i can connect a battery charger to, is that my only option?

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raymacke
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Re: Bo Jest: power questions, engine type

Post by raymacke » Sun May 05, 2013 8:50 pm

Tom, can better answer this but if I remember correctly the 9.9 Yamaha High Thrust (T model) comes with electric start AND the rectifier regulator and is set-up to charge the battery. The 9.9 portable "F" model does not have either. I believe what they are stating is if you want to use the alternator output on a model that DOES NOT have a pre-installed Rectifier Regulator (F model) the accessories used need to be able to tolerate 18V. IF the Rectifier Regulator is installed on one of these then all 12v accessories can be used.

What is going on is with the unregulated outboard the output voltages vary a lot and are not limited to the 15.5 or so voltage like the regulated systems - they can peak at 18+ volts. This high voltage will take a toll on maintenance free batteries as they are not as tolerant to high voltage charging. Also, some electronic equipment will safely operate on a wide range of voltage while others will not. This is usually called out in the specifications of the unit written something like "Required Voltage - 8.5v to 20.2v". As long as it states 18v+ it would be OK on the unregulated system. Personally, I would want the regulator and not have to worry about it.

TOM (or anyone else!), please correct me if I am wrong!
So Many Rivers,
So Little Time....

scout
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Re: Bo Jest: power questions, engine type

Post by scout » Mon May 06, 2013 9:12 am

here's the spec page for the 9.9 high thrust:

http://www.yamahaoutboards.com/outboard ... ifications

it shows alternator at 6A. -- but unlike the other listing (guess i was looking at wrong model) it does not have (rectifier/regulator required) following that number, so must be built in?

so, if it stays in the range for charge, should be good to go, i guess.

honestly -- this 12v battery power stuff gets a bit confusing. :shock:
sizing the battery for all that's running from it, etc., does require some thought!

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raymacke
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Re: Bo Jest: power questions, engine type

Post by raymacke » Mon May 06, 2013 6:23 pm

Yes, I fairly sure the High Thrust does have the regulator. Since it has electric start it would need a way to charge the battery.

With batteries bigger is usually better BUT bigger means more time and/or more amps to keep it charged. Definitely a balancing act.
So Many Rivers,
So Little Time....

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tsmitherman
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Re: Bo Jest: power questions, engine type

Post by tsmitherman » Tue May 07, 2013 2:10 am

The High Thrust model with electric start has a regulator and will charge the batteries fine. I'm not sure about the manual start model, but since you're going to have the need for batteries for other things, the electric start model is probably what you'll want anyway.

I use two small marine batteries (from Walmart) hooked together and the 6 amp alternator keeps them charged just fine. Of course we usually run 6-10 hours a day, which is more than others might. On the Knot-So-Fast, all my lights are LEDs, the two small fans in the V-berth draw 0.3 amps, and electronics consists of a Garmin chartplotter and VHF radio. So I don't have a need for lots of power, and the 9.9's alternator and two battery setup works fine for me.

We met a couple in Demopolis, AL last Fall who were on sailboat equipped with a Yamaha 9.9 HT, and they had a relatively large portable refrigerator (West Marine brand) onboard. The Captain told me they'd been cruising for a couple of months and even running the refrigerator full time, the 9.9 kept the batteries charged. I suspect that he also ran the engine all day long.
Tom
------------------------------
36' 1969 Willard Aft-Pilothouse Trawler
Blog: www.genesisboat.blogspot.com


Knot-So-Fast (BoJest) SOLD

"It's amazing what one can do when one doesn't know what one can't do." - Garfield

scout
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Re: Bo Jest: power questions, engine type

Post by scout » Tue May 07, 2013 8:38 am

Tom,

what your running is nearly what i'll be :)
led lites, a garmin chartplotter and a radio, and an engle fridge.
everything else will be off until hooked to shore power.

could i trouble you for the amp hours on your batteries?
if i'm not running as many hours per day, should i get a battery charger for when i'm at the dock?

thanks!

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